THE  PRINCIPLES  OF  BIOLOGY 


H  AM  A  KER 


PHE  PRINCIPLES  OF 
BIOLOGY 


BY 

J.  I.  HAMAKER,  PH.  D. 

PROFESSOR  OF  BIOLOGY,  RANDOLPH-MACON 
WOMAN'S  COLLEGE 


WITH  267  ILLUSTRATIONS 


PHILADELPHIA 

P.   BLAKISTON'S   SON   &   CO. 

1012  WALNUT  STREET 
1913 


BIOLOGY 

LIBRARY 

G 


COPYRIGHT,  1913,  BY  P.  BLAKISTON'S  SON  &  Co. 


THK-  MAPLE.  PRESS.  YORK.  PA 


PREFACE 


In  an  introductory  course  in  biology  a  text-book  is  helpful 
because  the  notes  taken  by  lower  class  students  are  usually 
unsatisfactory,  if  not  useless,  and  require  much  time.  This 
volume  contains  an  outline  of  a  course  given  by  the  author  for 
more  than  ten  years.  It  is  designed  to  supplement  the  practical 
work  in  the  laboratory  and  field,  and  to  relieve  the  student  of 
the  greater  part  of  the  burden  of  taking  lecture  notes.  Purely 
discriptive  matter  is  reduced  to  a  minimum  and  examples  are 
largely  omitted  because  the  teacher  is  supposed  to  have  suffi- 
cient command  of  the  subject  to  supply  these,  and  local  and 
familiar  examples  are  always  better  for  illustration  than  those 
less  well  known.  The  figures  in  the  book  are  regarded  as  an 
important  part  in  the  presentation  of  the  subject  and  should 
be  carefully  studied.  Many  points  omitted  or  only  briefly 
alluded  to  in  the  text  are  explained  by  them. 

Part  I  is  designed  to  acquaint  the  student  with  the  funda- 
mentals of  plant  organization  and  life  processes.  Part  II 
does  the  same  for  animals  but  in  a  different  and  more  thorough 
method.  Part  III  discusses  the  most  important  general  biolog- 
ical phenomena.  The  difference  in  treatment  of  plants  and 
animals  rests  on  well-known  pedagogical  principles  which 
need  not  be  discussed  here.  There  is  little  in  the  book  for 
which  originality  can  be  claimed,  but  justification  for  publishing 
rests  on  the  fact  that  there  is  at  present  no  text  which  even 
approximately  covers  the  field  in  subject-matter  and  method 
of  treatment. 

For  the  classification  of  plants  and  animals  given  in  the  ap- 
pendices to  Parts  I  and  II,  many  authorities  have  been  con- 


VI  PREFACE 

suited  but  none  followed  consistently.  In  the  interest  of  the 
beginner  simplicity  is  desirable  and  to  this  end  a  number  of 
unimportant,  obscure  or  aberrant  groups  have  been  omitted 
and  only  three  grades,  or  orders  of  groups,  recognized,  viz., 
Branch  or  Phylum,  Class,  and  Order.  The  classification  of 
animals  given  in  the  "Lehrbuch"  of  Claus-Grobben  has  been 
followed  more  closely  than  that  of  any  other  author. 

Acknowledgment  is  due  the  publishers,  Messrs.  P.  Blakis- 
ton^s  Son  &  Co.,  for  the  loan  of  many  figures  from  the  works 
of  Galloway,  Stevens,  Folsom  and  others.  The  Macmillan  Co. 
and  Henry  Holt  and  Co.  have  also  kindly  furnished  several 
figures.  More  specific  acknowledgment  is  made  in  connection 
with  each  borrowed  figure. 

J.  I.  HAMAKER. 
COLLEGE  PARK,  VA. 


TABLE  OF  CONTENTS 


INTRODUCTION 

PAGE 

Biology  and  the  biological  sciences       i 

The  living  and  the  not-living '.    .       2 

The  living  substance — protoplasm 4 

PART  I.— PLANTS 

Laboratory  and  field  exercises 7 

Color.     The  light  relation.     The  leaves.     Phyllotaxy.     The  stem. 

The  roots 15 

Seeds.  Germination.  The  seedling.  The  mature  plant.  Structure 
and  function  of  the  roots.  Structure  and  function  of  the  stem. 
Structure  and  function  of  the  leaves.  Photosynthesis.  Res- 
piration. Translocation  of  food  substances.  Other  food  sub- 
stances. Differentiation  of  tissues 22 

Modified  roots.     Modified  stems  and  branches.     Modified  leaves .    .     49 
Homology  of  the  flower.     Inflorescence.     Structure  of  the  flower. 
Function   of   the   flower.     Pollination   and   fertilization.     The 

seed.     The  fruit.     Seed  distribution 53 

Classes  of  plants :  Angiosperms,  Gymnosperms,  Cryptograms    ...      71 
Ecology:    Water,  temperature,  latitude  and  altitude,  light,  soil,  re- 
lation of  plants  to  each  other,  carnivorous  plants,  physiographic 
relations 75 

APPENDIX  TO  PART  I 
CLASSES  OF  PLANTS 

Branch  I.  Thallophyta       101 

Class    i.  Myxomycetes      101 

2.  Schizophyta 102 

3.  Diatomeae 105 

4.  Conjugate       105 

vii 


Vlll  TABLE    OF    CONTENTS 

PAGE 

5.  Chlorophyceae 106 

6.  Characeae 106 

7.  Phaeophyceae 107 

8.  Rhodophyceae      107 

9.  Phycomycetes 108 

10.  Basidiomycetes 108 

11.  Ascomycetes 109 

Lichenes no 

Branch  II.  Bryophyta  in 

Class  i.  Hepaticae in 

2.  Musci 112 

Branch  III.  Pteridophyta  113 

Class  i.  Filicinae 114 

2.  Equisetinae       115 

3.  Lycopodinae 115 

Branch  IV.  Spermatophyta       116 

Class    i.  Gymnospermae 117 

2.  Angiospermae 117 

PART  II.— ANIMALS 

Laboratory  exercises 119 

Introduction:  Color  and  form.  Locomotion.  Axis  of  locomotion. 
Cephalization.  Dorsal  and  ventral.  Right  and  left.  Bilateral 
symmetry.  Radial  symmetry.  Universal  symmetry.  Asym- 
metry. Exceptional  cases.  Size  and  differentiation.  Integu- 
ment. Nerve-muscle  Mechanism.  Digestion.  Circulation. 
Respiration.  Excretion.  Reproduction.  Organization  of  the 
body.  "Higher"  and  "lower"  animals.  Segmentation — meta- 
meres,  antimeres 127 

Integument:  General  integument  of  amoeba,  hydra,  worms,  arthro- 
pods, vertebrates.  Specialized  integumentary  structures — cu- 
ticular,  epidermal  and  dermal  structures,  glands 141 

Sense  organs:  General  sense  organs  in  amoeba,  hydra,  worms,  arthro- 
pods, vertebrates.  Organs  of  special  sense:  The  chemical 
senses — taste  and  smell.  The  organs  of  sight.  The  arthropod 
eye.  The  vertebrate  eye.  Types  of  vision.  Mechanism  for 
focusing  and  control  of  light  intensity.  Hearing  and  equilibra- 
tion. Statocysts.  The  ear — of  arthropods,  of  vertebrates. 
The  senses  of  lower  animals 151 


TABLE    OF    CONTENTS  IX 

PAGE 

Organs  of  response:    In  amoeba,  hydra,  annelids,  arthropods,  verte- 
brates.    Skeleton  and  connective  tissue.     The  endoskeleton  of 

vertebrates.      Muscular  action 172 

The  nervous  system:  Ccelenterates.     Annelids.     The  mechanism  of  re- 
sponse.    Arthropods.     Vertebrates.     The  brain  and  spinal  cord. 

A  spinal  nerve.     The  cranial  nerves 182 

Energy  relations  of  the  animal:  The  food  of  animals  a  source  of 
energy.     Digestion   in   amoeba.     Fermentation   and   digestion. 
Digestion  in  ccelenterates,  in  worms,  in  arthropods.     The  diges- 
tive tract  of  vertebrates.     Digestive  ferments.     Absorption.  .    .    191 
Circulation:  The  gastro-vascular  cavity  of  ccelenterates.     Circulation 

in  worms,  in  arthropods,  in  vertebrates.     The  lymphatic  system.  204 
Respiration:  in  minute  animals,  in  aquatic  animals,  in  insects,  in 

vertebrates.     The  blood  as  respiratory  vehicle 208 

Metabolism:  growth,  secretion  and  excretion,  muscular  activity   .    .    213 
Excretion:  in  minute  animals,  in  worms,  in  crayfish,  in  vertebrates   .   215 
Reproduction:     amoeba,  conjugation,  hydra,  annelids,  crayfish,  verte- 
brates      217 

APPENDIX  TO  PART  II 

CLASSES  OF  ANIMALS 

Phylum    i. — Protozoa:  Classes  of  protozoa 224 

Phylum    2. — Ccelenterata:  Porifera.     Structure  of  a  sponge.     Hy- 

drozoa.     Scyphozoa.     Anthozoa.     Ctenophora 230 

Phylum    3. — Scolecida:   Platyhelminthes.     Aschelminthes.  Nemer- 

tini 224 

Phylum    4. — Annelida:  Classes  of  annelids    .    . 247 

Phylum    5. — Molluscoidea :  Bryozoa.     Brachiopoda 248 

Phylum    6. — Echinodermata:    Pelmatozoa.     Asteroidea.     Structure 

of  star  fish.     Ophiuroidea.     Echinoidea.     Holothuroidea  ...   250 
Phylum    7. — Arthropoda:  Branchiata.     The    orders    of    Crustacea. 
Palaeostraca.  Arachnoidea.   Protracheata.   Myriapoda.  Aptery- 
gogenea.     Insecta.     Structure   of   an   insect.     The    orders   of 

insects 259 

Phylum    8. — Mollusca:  Amphineura.     Conchifera.     Orders  of  Con- 
chifera.     Structure  of  a  snail.    Structure  of  a  clam.    Structure  of 

of  a  squid 275 

Phylum    9. — Adelochorda      284 


X  TABLE    OF    CONTENTS 

PAGE 

Phylum  io. — Urochorda.  Classes  of  tunicates 285 

Phylum  ii. — Acrania  286 

Phylum  12. — Vertebrata:  Cyclostorhata.  Pisces.  Orders  of  fishes. 

Amphibia.     Orders  of  amphibia.     Reptilia.     Orders  of  reptiles. 

Aves.     Orders  of  birds.     Mammalia.     Orders  of  mammals   .    .    287 

PART  III.— GENERAL  PRINCIPLES 

Spontaneous  generation.  Continuity  of  the  living  substance.  Struc- 
ture of  protoplasm.  The  nucleus.  Chemical  structure  of 
protoplasm.  Function  of  cytoplasm  and  nucleus.  Cell  division. 
Number  of  chromosomes.  Nucleoli.  Centrosomes.  Spindle 
fibres.  Resting  nucleus.  Conjugation.  Fertilization.  Matur- 
ation. Conjugation  in  protozoa.  Fertilization  stimulus.  Cleav- 
age. The  blastula.  The  gastrula.  The  medullary  plate.  The 
notochord.  The  mesoderm.  Other  types  of  cleavage.  Origin 
of  the  tissues.  Indirect  development.  Differentiation  of  germ- 
inal and  somatic  tissues.  Division  of  labor  and  differentiation. 
Regeneration.  Mechanics  of  growth.  Progressive  and  regressive 
development.  Sexual  dimorphism.  Polymorphism.  Alterna- 
tion of  generations.  Life  habits  depending  on  food.  Parasitism. 
Protozoa  as  parasites.  Bacteria  as  parasites.  Immunity  .  .  .  307 

Species.  Variation.  Heredity.  Mendel's  law.  Physical  basis  of 
heredity.  Number  of  species.  Origin  of  species.  The  Taxo- 
nomic  Series.  The  Phylogenetic  Series.  The  Ontogenetic  Series. 
The  struggle  for  existence.  Natural  Selection.  Animals  and 
plants  under  domestication.  Geographical  distribution.  382 

Adaptations:  Pollination.  Care  of  young.  Sexual  dimorphism. 
Sexual  selection.  Welfare  of  the  individual  and  of  the  species. 
Animal  coloration.  Protective  resemblance.  Feigning.  Mim- 
icry. Color  changes.  Luminescence.  Electrical  organs.  In- 
stinct. Intelligence 40*; 

Index ( 


THE  PRINCIPLES  OF  BIOLOGY 


INTRODUCTION 

1.  Biology  is  the  science  which  treats  of  living  things,  or  of 
objects  having  life.     In  the  broad  application  of  the  term,  as 
used  here,  Biology  includes  a  number  of  more  special  sciences. 
Morphology,  Anatomy  and  Histology  treat  of  form  and  struc- 
ture.    On  the  other  hand,  Physiology  deals  primarily  with  the 
function  of  organs.     In  Embryology  it  is  the  development  of 
the  individual,  especially  during  the  earlier  stages,  that  is  kept 
chiefly  in  view.    Paleontology  treats  only  of  fossils,  that  is,  those 
types  of  living  things  which  existed  at  some  earlier  period  in 
the  world's  history  but  which  have  now  become  extinct. 

2.  Even  with  such  a  sub-division  of  the  subject  we  have 
left  special  sciences  which  cover  such  a  broad  field  that  they 
become  unwieldly.     In  Botany   and  Zoology  the  subject  is 
divided  on  the  basis  of  the  kinds  of  living  things  considered, 
the  former  being  the  biology  of  plants,  the  latter  the  biology 
of  animals.     Still  further  sub-division  leads  to  Cryptogamic 
Botany,    Phanerogamic    Botany,    Invertebrate   Zoology    and 
Vertebrate  Zoology.     Bacteriology,  Entomology  (insect  zool- 
ogy), Ornithology  (bird  zoology),  and  still  other  more  narrowly 
restricted  branches  of  biology  are  recognized.     The  very  ex- 
tensive study  of  man  has  given  rise  to  a  number  of  biological 
sciences  dealing  only  with  this    single    genus;  viz.,   Human 
Anatomy,  Human  Physiology,  Human  Embryology,  Anthro- 
pology (dealing  with  the  comparative  anatomy  of  the  various 


2  INTRODUCTION 

races  of  men),  and  Ethnology  (dealing  with  manners,  customs, 
language  and  other  activities  of  the  races  of  men). 

3.  Biology  deals  with  objects,  that  is,  with  the  concrete, 
but  its  chief  interest  and  value  lies  not  in  mere  description  or 
enumeration  so  much  as  in  the  generalizations,  which  may  be 
made    from    accumulated    facts.     A    single    observation,    or 
repeated  observation  of  a  single  individual  seldom  justifies  a 
general  conclusion,  but  by  the  comparison  of  numerous  examples 
one  is  enabled  to  distinguish  the  accidental  and  trivial  from  the 
general  and  significant.     Therefore,  the  method  of  study  by 
comparison  is  for  the  biologist  of  special  importance. 

(For  the  individual,  and  especially  for  the  average  college 
student  who  devotes  a  comparatively  short  time  to  the  subject, 
the  study  of  many  individual  examples  is  impossible  and,  there- 
fore, in  practice,  an  abridgement  of  the  method  of  study  by 
comparison  is  adopted.  This  is  called  the  method  of  study  by 
types,  by  which  a  series  of  examples  are  compared,  each  example 
being  representative  or  typical  of  a  considerable  group.  That 
the  examples  selected  are  typical,  rests,  of  course,  on  the 
observation  of  previous  students  or  investigators.  By  this 
method  the  student  may  in  a  comparatively  brief  time  extend 
his  studies  over  a  large  field.) 

4.  It  is  usually  not  difficult  to  distinguish  a  living  thing  from 
one  not  living,  but  to  state  formally  what  are  the  attributes 
of  the  living  is  not  so  simple  a  matter.     On  close  analysis 
living  things   are  found   to  be   complex  in  structure,   being 
composed  of  many  parts,  called  organs,  which  differ  in  struc- 
ture and  in  function.     For  this  reason  a  living  thing  is  called 
an  organism,  and  is  said  to  be  organized. 

5.  Crystals  are  more  like  organisms  than  any  other  non- 
living thing,  and  a  comparison  of  organisms  and  crystals  will 
serve  to  indicate  the  most  essential  characteristics  of  living 
things. 

6.  One  of  the  most  prominent  characteristics  of  objects  that 


CRYSTALS   AND   ORGANISMS  3 

have  life  is  growth:  crystals  also  grow,  but  not  in  the  same 
way.  The  method  of  growth  in  crystals  is  by  accretion,  i.  e., 
by  addition  of  substance  to  the  outside;  but  in  the  growth  of 
living  organisms  the  added  substance  is  taken  up  into  the 
interior  of  the  body,  i.  e.,  by  intussusception.  Moreover,  in 
crystals  the  chemical  nature  of  the  substance  added  is  not 
altered,  while  in  the  case  of  organisms  the  substance  added 
undergoes  a  series  of  chemical  changes  before  it  is  finally 
really  part  of  the  growing  body.  This  process  of  transforming 
food  material  is  called  assimilation  and  is  wholly  wanting  in 
crystals. 

7.  Another  prominent   characteristic  of    organisms  is   the 
definiteness  of  the  shapes  which  they  assume.     Each  plant  or 
animal  is  as  much  like  every  other  individual  of  the  same 
kind  as  if  they  were  all  made  after  the  same  pattern  or  cast 
in  the  same  mold.     Crystals  are  bounded  by  plane  surfaces 
which  meet  at  definite  angles  but  within  this  limitation  the 
shape  may  vary  indefinitely. 

8.  The  size  of  organisms  is  limited.     The  individual  grows 
more  or  less  rapidly  until  it  reaches  a  certain  size  after  which 
growth  almost  or  wholly  ceases.     The  size  of  crystals  has  no 
definite  limitations. 

9.  Crystals  may  be  formed  under  favorable  circumstances 
wherever  the  substances  of  which  they  are  composed  are  found. 
But  we  have  no  knowledge  that  a  living  thing  is  ever  formed 
under  any  conditions  except  by  development  from  what  we 
may  call  a  germ,  which  came  from  some  pre-existing  living 
thing,  and  which  differs  from  the  mature  organism  chiefly  in 
being  smaller  and  simpler  in  structure.     The  crystal  may  be 
reduced   to   its   constituent  elements  which  will  again  unite 
under  the  proper  conditions  to  form  a  new  crystal,  whereas  if  the 
organism  is  similarly  reduced,  its  elements  will  under  no  condi- 
tions recombine  to  produce  a  new  organism. 

10.  Crystals  may  exist  indefinitely,  but  the  life  of  the  in- 


4  INTRODUCTION 

dividual  organism  is  limited.  It  comes  to  an  end  by  death  and 
subsequent  decay,  by  division,  or  by  fusion  of  its  body  with 
another  similar  body.  In  either  case  the  living  individual,  as 
such,  ceases  to  exist. 

11.  Crystals  are  inert,  while  organisms  possess  to  some  degree 
the  power  of  movement  in  response  to  an  external  stimulus. 

12.  Thus  we  have  the  material  world  made  up  of  lifeless,  or 
inorganic  bodies,  and  living,  or  organic  bodies.     The  following 
table  exhibits  in  parallel  columns  the  similarities  and  differences 
of  the  two  classes  of  bodies: 

CRYSTALS  ORGANISMS 

1.  Are  unorganized.  i.  Are  organized. 

2.  Grow    by     accretion     (so  2.  Grow  by  assimilation  and 
also  hailstones,  concretions,  intussusception, 
stalactites). 

3.  Have  indefinite  shape  and  3.  Have   definite   shape   and 
plane  surfaces.  curved  surfaces. 

4.  Size  not  limited.  4.  Size  limited. 

5.  Generate  spontaneously.  5.  Develop  from  a  germ. 

6.  May  exist  indefinitely.  6.  Have  a  limited  life  period. 

7.  Are  inert.  7.  Have  power  of  motion. 

THE  LIVING  SUSBSTANCE 

13.  All  the  activities  of  an  organism,  by  which  it  is  distin- 
guished from  inorganic  bodies,  are  the  activities  of  the  living 
substance,  which  is  called  protoplasm.     But  not  all  of  the  sub- 
stance of  an  organism  is  protoplasm.     Besides  the  protoplasm 
there  is  usually  more  or  less  inert  substance  which  was  formed 
by  the  protoplasm,  but  which  does  not  of  itself  possess  life. 
Of  such  substances  are  the  hard  parts  of  bones  and  the  super- 
ficial layers  of  the  skin,  the  corky  layers  of  bark  and  the  hard 
fibres  of  wood,  etc.     This  inert  substance  may  be  wholly  want- 
ing, or  it  may  constitute  the  larger  part  of  the  body  of  the 
organism. 


PROTOPLASM 


14.  Besides  the  protoplasm  and  the  inert  substances  formed 
by  it,  there  are  in  many  cases  foreign  substances  to  be  found 
within  the  various  organs  of  an  organism; 

such  are,  for  example,  the  pebbles  found  in 
the  gizzard  of  certain  birds,,  the  particles  of 
sand  found  in  the  antennal  organ  of  the 
crayfish,  or  the  sand  used  by  many  minute 
animals  in  forming  a  skeletal  shell  or  test. 

15.  Protoplasm  is  jelly-like  in  consis- 
tency, and  transparent,  but  not  perfectly 
homogeneous.      Under  the  microscope  it 
is  seen  to   consist  of  an  infinitely  large 
number  of  minute  particles  of  various  sizes 
and  of  different  optical  and  chemical  char- 
acteristics.    Chemical  analysis  shows  that 
it  is  highly  complex;  consisting  largely  of 
carbon,  oxygen,  hydrogen  and  nitrogen, 
with  small  quantities  of  sulphur,  and  occa- 
sionally phosphorous,  manganese,  magnesium,  calcium,  sodium, 
and  chlorine.     It  is  regarded  as  a  more  or  less  definite  aggre- 
gation of  a  large  number  of  chemi- 
cally complex  bodies. 

1 6.  In  the  smaller,  microscopic 
organisms  the  protoplasm  may 
usually  be  observed  to  consist  of  two 
parts,  nucleoplasm  and  cytoplasm. 
The  nucleoplasm,  in  the  form  of  a 
round  or  oval  body,  the  nucleus, 
occupies  the  centre  of  the  mass  and 
is  surrounded  by  the  cytoplasm. 
The  nucleus  and  the  surrounding 
cytoplasm  together  are  called  a  cell. 

In  larger  organisms  there  are  a  large  number  of  nuclei  quite 
regularly  distributed  throughout  the  protoplasm.      There  are 


FIG.  i.— The  test  of 
a  protozoan,  Difflugia, 
composed  of  minute 
grains  of  sand  cemented 
together. 


FIG.  2. — Diagram  of  the  cell. 
For  details  see  Fig.  179. 


6  INTRODUCTION 

then  as  many  cells  as  there  are  nuclei,  and,  frequently,  each 
cell  is  marked  off  from  its  neighbors  by  a  wall  of  inert  substance 
secreted  by  the  protoplasm.  The  character  of  this  wall  varies, 
not  only  with  the  kind  of  organism,  but  also  with  the  organ  in 
which  it  is  found.  In  the  central  part  of  a  tree  trunk  the  thick 
cell  walls  form  the  firm  substance  of  wood;  similarly,  near  the 
surface  they  form  the  corky  layers  of  the  bark. 

17.  What  has  been  said  in  the  preceding  paragraphs  has 
a  general  application  to  all  organisms,  but  the  obvious  grouping 
of  living  things  into  two  kingdoms — the  vegetable  and  the 
animal — is  based  on  certain  distinctive  peculiarities  which  are 
of  such  far-reaching  significance  and  which  separate  the  more 
familiar  forms  of  the  two  groups  so  precisely  that  it  will  be  con- 
venient to  study  each  group  separately.  Plants  are  on  the 
whole  much  simpler  than  animals  and  therefore  better  adapted 
for  introductory  study. 


PART  l.-PLANTS 

LABORATORY  AND  FIELD  EXERCISES 

PRELIMINARY  SURVEY 
I.  Light  Relation  of  Leaves 

An  erect  stem  with  opposite  leaves  (Coleus). 

a.  Horizontal  view  showing  stem  (nodes  and  internodes)  and  leaves. 

b.  Make  a  diagram  showing  arrangement  of  leaves  as  seen  from 
above.     How  many  vertical  ranks  are  there? 

An  erect  stem  with  whorled  leaves  (Galium).     Horizontal  view  as 

in  la. 

An  erect  stem  with  alternate  leaves  (Quercus). 

a.  Horizontal  view  as  in  za. 

b.  Diagram  as  in  ib. 

Leaf  Rosette  (Plantago)  as  in  ib. 

Horizontal  stem  with  alternate  leaves  (Castanea).     Vertical  view 

showing  carefully  how  each  leaf  is  connected  with  the  stem. 

Horizontal  stem  with  alternate  leaves  (Tropaeolum) .     Horizontal 

view.     Show  relation  of  leaves  with  stem  as  in  5. 

Leaf  Mosaic  (Ampelopsis  Veitchii)  seen  from  the  direction  of  the 

midday  sun. 

Inverted  stems. 

a.  Wistaria.     Relation  of  leaves  to  stem  (note  the  pulvinus). 

b.  Salix  Babylonica.     Relation  of  leaves  to  stem. 

H.  Form  of  the  Plant  as  Related  to  Light 

a.  Form  of  a  tree  in  an  open  space  (Pinus). 

b.  Form  of  same  kind  of  tree  growing  in  a  thicket.     Note  difference 
in  lower  branches  in  a  and  b  and  explain. 

An  excurrent  type  of  stem  (Populus). 
A  deliquescent  type  of  stem  (Ulmus). 

7 


8  PLANTS 

HI.  Phyllotaxy 

12.  Recall  the  opposite  and  whorled  types  of  phyllotaxy. 

13.  a.    Arrangement  of  leaves  in  grasses  (Zea  Mays). 

b.  Arrangement  of  leaves  in  sedges  (Carex). 

c.  Arrangement  of  leaves  in  sour  wood  (Oxydendron),  oak  (Quer- 
cus),  etc. 

d.  Arrangement    of    leaves    in    mullein    (Verbascum),    goldenrod 
(Solidago),  Easter  lily  (Lilium  Harrisii). 

e.  Arrangement  of  leaves  (scales)  in  pine  (Pinus). 

IV.  Morphology  of  the  Leaf 

14.  Structure  of  a  typical  simple  leaf  (Pyrus).     Identify:  blade,  petiole, 
stipules  and  (in  the  blade)  midrib,  veins,  and  veinlets. 

15.  Types  of  venation:  Reticulate — a.  Pinnate  (Castanea),  b.  Palmate 
(Ampelopsis  Veitchii).     Parallel — c.  Basal  (Convallaria),  d.  Costal 
(Canna). 

16.  Form  of  the  margin  of  simple  leaves.     Find  examples  among  the 
leaves    already    studied    of — a.  Entire,    b.  Serrate,    c.  Lobed.     In 
addition   draw   one   that   is   d.  Parted    (Ricinus),  and  e.  Divided 
(Bidens). 

17.  Compound  leaves:  a.  Pinnately  compound  (Robinia) — note  rachis 
and  leaflets,     b.  Palmately  compound  (Parthenocissus). 

1 8.  Structure  of  the  blade,     a.  With  the  edge  of  the  scalpel  strip  off  the 
thin  membrane  (epidermis)  covering  the  upper  and  lower  surfaces 
of  the  blade  (Caladium).     Study  the  epidermis  with  a  hand  lens  and 
with  a  needle  determine  the  texture,     b.  With  the  lens  study  both 
surfaces  of  the  green  part  of  the  blade  (mesophyll)  where  the  epi- 
dermis has  been  removed.     The  denser  portion  is  palisade  mesophyll, 
the  other,  spongy  mesophyll.     Determine  texture  with  the  needle. 

c.  Scrape  out  some  of  the  mesophyll  and  soak  it  for  a  time  in  alcohol 
in  a  test-tube.     Note  the  result.     The  green  matter  is  chlorophyll. 

d.  Can  you  find  pores  (stomata)  in  the  epidermis? 

V.  Stems  and  Roots 

19.  Study  a  cross  section  of  a  stem  (Paulo wnia)  (Sambucus).     Determine 
the  texture  of  the  wood,  pith  and  bark. 

20.  Study  a  similar  stem  in  longitudinal  section  in  the  region  of  the 
node,  as  in  19. 

21.  Compare  the  root  of  a  similar  plant  with  the  stem,  as  in  19  and  20. 


LABORATORY  EXERCISES  9 

THE  LIFE  HISTORY  OF  A  PLANT 
VI.  THE  SEED 

A  dicotyledonous  seed  (Phaseolus). 

a.  Draw  two  views  of  a  bean  seed  to  show  the  general  form,  the 
hilum,  the  micropyle  and  the  chalaza. 

b.  From  a  seed  that  has  been  soaked  in  water  remove  the  seed 
coats  (testa  and  tegmen).     Study  the  coats. 

c.  Draw  the  embryo,  showing  the  cotyledon  and  caulicle. 

d.  Separate  the  cotyledons  and  draw  to  show  the  cotyledons,  the 
plumule  and  the  caulicle. 

Other  dicotyledonous  seeds: 

a.  Study  a  seed  of  Pisum  as  in  220,  b,  c,  and  d. 

b.  Compare  seeds  of  Trifolium  and  Raphanus  (or  Brassica)  with 
those  of  Phaseolus  and  Pisum. 

c.  Study  a  seed  of  Cucurbita  as  in  220,  b,  c,  and  d. 
A  monocotyledonous  seed  (Zea  Mays). 

a.  Draw  that  side  of  a  grain  of  corn  which  shows  the  embryo. 

b.  Remove  the  seed  coats  from  a  specimen  which  has  been  softened 
in  water  and  find  the  embryo  embedded  in  the  mealy  endo- 
sperm.    Study  the  seed  coats,  but  do  not  try  to  homologize 
with  those  of  the  bean.     They  are  more  complex. 

c.  With  a  sliding  cut  make  a  longitudinal  section  of  a  seed  through 
the  shorter  diameter  so  as  to  exactly  halve  the  embryo.     Draw 
the  section  and  identify  cotyledon,  plumule  and  caulicle. 

d.  Cut  another  transversely  at  three  points,  so  as  to  divide  the 
embryo  into  quarters.     Draw  the  three  sections. 

e.  Compare  a  seed  of  Triticum  with  that  of  Zea. 

A  seed  of  a  Gymnosperm  (Pinus).     Study  a  pine  nut  noting  the 
character  of  the  seed  coat,  endosperm  and  embryo. 

VII.  Development  of  the  Seedling 

Where  in  a  germinating  bean  does  evidence  of  growth  first  appear? 

Where  and  how  does  the  developing  part  first  emerge  from  the 

seed  coats?     Compare  plumule  and  caulicle  with  regard  to  rate  of 

development  during  the  first  week  of  growth. 

Compare  pea,  squash  and  corn  with  bean  in  regard  to  each  point 

mentioned  in  26. 

How  does  each  of  the  four  kinds  of  seedlings  emerge  from  the  soil? 


10  PLANTS 

29.  Compare  the  cotyledons  of  the  seeds  in  regard  to  their  behavior 
during  the  early  stages  of  development. 

30.  Why  does  the  primary  root  grow  downward?     To  be  determined 
by  experiment  i. 

Experiment  i. — Spread  a  piece  of  moist  white  filter  paper  in  the  bottom 
of  a  shallow  plate  or  pan.  On  this  set  a  bottle  about  three  inches  high 
with  a  cork  projecting  from  the  neck.  Fasten  several  pea  seeds  to  the 
cork  with  long  pins  in  such  a  way  that  they  will  be  suspended  in  mid-air 
at  least. an  inch  from  the  cork.  The  pins  may  be  thrust  through  the 
seed  coats  or  the  cotyledons  but  the  caulicle  and  plumule  must  not  be 
injured.  The  seeds  should  be  fastened  so  that  the  caulicle  is  directed 
downward  in  one  case,  upward  in  another  and  horizontally  in  another. 
Cover  the  whole  with  a  bell  jar  and  note  the  direction  taken  by  the  caulicle 
when  it  germinates.  Interpret  the  result.  See  paragraph  49. 

31.  Why  does  the  stem  grow  upward?    To  be  determined  by  experi- 
ment 2. 

Experiment  2. — Seal  up  the  hole  in  the  bottom  of  a  four  inch  flower  pot. 
Fill  the  pot  with  earth  or  sand  and  plant  some  pea  seeds  about  an  inch 
below  the  surface.  Moisten  the  soil  and  then  cover  the  pot  with  wire 
mosquito  netting  so  that  the  earth  will  not  fall  out  when  the  pot  is  in- 
verted. Invert  the  pot  and  support  it  on  an  empty  glass  tumbler  and 
cover  the  whole  with  a  tall  bell  jar.  The  tumbler  should  be  set  in  a  shallow 
dish  or  pan  on  a  piece  of  wet  white  filter  paper.  After  a  period  of  about 
ten  days,  carefully  raise  the  pot,  allowing  the  soil  to  fall  away  and  expose 
the  seedlings.  Interpret  the  result.  See  paragraph  50. 

32.  What  changes  occur  in  the  cotyledons  and  what  is  their  ultimate 
fate?    What  is  the  function  of  the  cotyledons? 

33.  The  young  plant — a.  bean,  b.  pea,  c.  squash,  d.  corn. 

34.  Reviewing  development  as  if  in  a  moving  picture,  describe  the  devel- 
opment of  the  plumule. 

35.  As  in  34  describe  the  development  of  the  caulicle. 

36.  The  hypocotyl  is  that  part  of  the  stem  which  develops  from  the 
caulicle.     Compare  the  hypocotyl  of  bean,  pea,  squash  and  corn. 

37.  Root  hairs  (wheat).     Study  under  glass  cover. 

38.  Draw  root  system  of  well  developed  bean.     Note,  primary,  or  tap 
root,  and,  secondary,  or  lateral  roots. 

39.  The  cotyledons  are  the  first  leaves.     Are  the  leaves  that  develop 
next  like  those  of  the  mature  plant?     Are  the  third?     Fourth? 
Compare  bean,  pea  and  squash. 

40.  Where  do  the  branches  appear?     See  also  section  viii. 


LABORATORY  EXERCISES  II 

Vm.  The  Mature  Plant 

The  root  (Quercus  alba)  in  cross  section.  Draw  x$.  Study  the 
details  carefully  with  hand  lens.  Is  there  a  central  pith  (medulla)  ? 
Medullary  rays?  Vessels  or  tracheae  in  the  wood.  How  arranged? 
Components  of  the  bark? 

With  scalpel  and  needles  determine  the  texture  of  the  various  parts 
of  the  root. 

Are  the  vessels  true  tubes  ?     Can  you  blow  through  them ?     (Exp.  3 .) 
Longitudinal  section  of  root  tips  (microscope,  prepared  slide) .     Note 
the  arrangement  of  the  cells,  and  the  root  cap. 
The  stem  (Quercus  alba)  (Aristolochia)  in  cross  section.     Draw  X4. 
a.     One  year  old  stem.     b.     Two  year  old  stem.     c.     Three  year 
old  stem.     Note  annual  rings,  medullary  rays,  epidermis,  cork, 
chlorophyll.     Compare  in  the  three  sections,  the  pith,  the  wood 
and  the  bark.     What  changes  occur  as  the  stem  grows  older? 
A  branch  at  least  three  years  old.     Can  you  determine  from  surface 
appearance  the  limits  of  the  i,  2  and  3  year  old  parts?     How?    Lo- 
cate the  limit  of  the  last  season's  growth  (the  twig). 
Draw  the  twig  (white  oak)  showing  the  scale  leaf  scars,  foliage  leaf 
scars,  terminal  and  lateral  buds.     What  is  the  function  of  these 
buds?  (see  48).     Are  there  any  branches?  (see  48). 
Study  the  growth  of  the  preceding  season.     Are  there  any  branches? 
How  old  are  they?    Where  do  they  occur?     What  determines  the 
position  of  a  branch  on  a  stem? 

How  old  is  the  basal  portion  of  your  branch?  Determine  by  surface 
inspection. 

Dissect  a  bud  (Hicoria  or  ^Esculus).  Note  the  character  of  the  bud 
scales  and  their  arrangement.  What  is  their  funtion?  What  do 
you  find  in  the  center  of  the  bud? 

Structure  of  the  stem  (white  oak).  Study:  a.  cross,  b.  longitudinal 
radial  and  c.  longitudinal  tangential  sections  of  a  block  at  least 
eight  years  old.  Note  epidermis,  cork,  chlorophyll,  other  tissues 
of  the  bark.  What  is  the  "grain"  of  wood?  What  are  the  flakes 
in  quartered  oak?  Why  must  the  wood  be  "quartered?" 
Compare  the  vessels  of  the  root  and  stem.  Compare  roots  and 
stems  of  the  same  diameter  with  regard  to  rigidity. 
Structure  of  the  stem  (corn),  a.  Study  cross  section  of  the  corn 
stem  noting  carefully  the  arrangement  of  the  pith  and  vascular 
bundles,  b.  Split  the  stem  and  draw.  Are  the  vascular  bundles 
continuous?  c.  Set  a  section  of  stem  in  red  ink  (Exp.  4).  Does 


12  PLANTS 

the  ink  rise  in  the  vascular  bundles?  d.  A  prepared  slide  under 
the  hand  lens  showing  the  vascular  bundles  and  the  vessels.  Draw 
one  bundle  X25. 

54.  The  leaf:  a.  Surface  view  of  the  epidermis  of  a  leaf  showing  the  sto- 
mata   (prepared  slide,  microscope),     b.  Cross  section  of  the  leaf 
(prepared  slide,  microscope).     Identify  the  layers  found  in  18. 

IX.  MODIFIED  STRUCTURES 
Modified  Roots 

55.  Fibrous  roots  (grasses). 

56.  Enlarged  (storage)  roots. 

(a)  Enlarged  tap-root  (turnip). 

(b)  Enlarged  fascicled  roots  (Dahlia). 

(c)  Enlarged  lateral  roots  (sweet  potato). 

57.  Prop  roots  (corn). 

58.  Aerial  roots  as  holdfasts  (ivy). 

Modified  Stems  and  Branches 

(In  each  case  note  the  nodes  and  internodes  and  the  character  of  the 
leaves  and  buds.) 

59.  Procumbent  stems  (periwinkle). 

60.  Runner  or  stolon  (strawberry). 

61.  Underground  stems: 

(a)  Rootstock  (Smilax). 

(b)  Rhizome  (Solomon's  seal). 

(c)  Tuber  (Irish  potato). 

(d)  Corm  ( Jack-in- the-pulpit). 

62.  Climbing  stems: 

(a)  Twining  stems  (morning  glory) . 

(b)  Climbing  by  spiral  tendrils  (grape). 

(c)  Climbing  by  adhesive  tendrils  (Virginia  creeper). 

(d)  Climbing  by  aerial  roots  (ivy). 

63.  "Stemless"  plants: 

(a)  Study  a  plant  of  salsify.     Is  there  a  stem?     Where  does  the  root 
begin?     Note  the  leaf  scars. 

(b)  Make  a  longitudinal  section  of  root  and  stem.     Note  distribution 
of  pith  and  vascular  bundles. 

(c)  Make  cross  sections  of  the  stem  and  the  root.     Note  again  as  in  b. 

(d)  Compare  a  beet  or  turnip  with  the  salsify  as  in  a,  b  and  c. 


LABORATORY  EXERCISES  13 

Storage  stems  (cactus).     What  is  stored?     Note  the  condition  of 
the  leaves.     Where  is  the  chlorophyll? 

Cladophylls  (Myrsiphyllum).     What  are  the  small  scalelike  struc- 
tures borne  by  the  stems  ?    What  are  the  leaf-like  organs  (cladophylls) 
borne  in  the  axils  of  the  scales? 
Thorns: 

(a)  What  are  the  thorns  on  the  black  locust? 

(b)  What  are  the  thorns  on  the  holly  and  barberry? 

(c)  What  are  the  thorns  on  the  honey  locust? 

Tendrils:  Compare  the  tendrils  of  the  grape  and  Virginia  creeper. 
Why  are  they  branches?     Note  how  the  grape  tendril  is  coiled. 

Modified  Leaves 

Scale  leaves.     Recall  various  types  already  studied. 

Tendrils.     Compare  leaves  of  the  pea,  vetch  and  vetchling.     What 

are  the  tendrils  in  each? 

Thorns.     See  paragraph  66,  a  and  b. 

Storage  leaves: 

(a)  Compare  leaves  of  Portulaca  and  the  houseleek. 

(b)  Make  a  longitudinal  section  of  an  onion.     What  are  the  scales? 
(Where  is  the  stem?) 

Traps: 

(a)  The  sundew  (Drosera).     Note  how  the  hairs  on  the  leaves  react 
to  contact  with  a  gnat  or  other  small  insect. 

(b)  The  pitcher  plant  (Sarracenia) .     What  devices  are  employed  for 
catching  insects? 

(c)  The  Venus  flytrap  (Dionaea). 

X.  Flowers 

A  simple  type  of  regular  flower  (Oxalis). 

(a)  Make  a  diagram  of  the  flower  as  it  would  appear  in  longitudinal 
section. 

(b)  Make  a  "plan"  diagram  to  show  how  the  parts  are  arranged 
around  the  center. 

(c)  Draw  one  member  of  each  cycle.     For  terms  see  text. 

(d)  Make  a  cross  section  of  the  ovary. 

A  simple  type  of  irregular  flower  (Swainsona).     Study  as  in  73. 
Study  other  more  modified  types  of  flowers  such  as  Primula,  etc. 


14  PLANTS 

XI.  Fruits 

76.  Simple  fruits;  dry,  dehiscent. 

(a)  A  follicle  (milkweed). 

(b)  A  legume  (bean). 

(c)  A  pod  (Yucca). 

77.  Simple  fruits;  dry,  indehiscent. 

(a)  A  samara  (maple). 

(b)  An  achene  (sunflower). 

(c)  A  caryopsis  (wheat). 

(d)  A  nut  (oak). 

78.  Simple  fruits,  fleshy: 

(a)  A  drupe  (plum). 

(b)  A  pome  (apple). 

(c)  A  berry  (cranberry,  persimmon). 

79.  Aggregate  fruits  (Magnolia). 

80.  Multiple  fruits  (pineapple). 

XII.  Classes  of  Plants 

For  distinguishing  characters  see  pages  71  ff.  and  101  ff. 

81.  Dicotyledons. — Make  a  list  of  at  least  ten  common  dicotyledonous 
plants. 

82.  Monocotyledons. — Make   a   list   of   at  least   ten  common    mono- 
cotyledonous  plants. 

83.  Gymnospermae. — Make  a  list  of  all  the  kinds  of  Gymnosperms  grow- 
ing in  your  vicinity. 

84.  Pteridophytes. — Dig  up  a  fern  with  all  the  roots  and  wash  away  the 
soil.     Draw  to  show  roots,  rootstock  and  leaves.     Study  the  under 
surface  of  a  fruiting  frond  with  the  lens. 

85.  Bryophytes. — Musci — Collect  a  number  of  kinds  of  moss.     Find 
plants  with  and  without  a  spore  capsule  but  otherwise  alike.     Draw 
both  kinds. 

86.  Bryophytes. — Hepaticae — Collect  and  study  liverworts  as  in  85. 

87.  Lichenes. — Collect  several  kinds  of  lichens. 

88.  Algae. — Collect  several  kinds  of  algae. 

89.  Fungi. — Collect  one  or  more  kinds  of  each  of  the  following  fungi: 

(a)  Mushrooms,  toadstools,  puffballs,  rusts  and  smuts. 

(b)  Mildews,  blue  and  green  molds  and  black  fungi. 

(c)  Water  molds  and  black  molds. 


COLOR   OF  PLANTS  15 

Color 

1 8.  Plants  are  usually  green.     This  is  so  commonly  true 
that  perhaps  the  most  general  idea  associated  with  the  term 
plant  is  that  of  the  green  color.     There  are,  however,  many 
plants  that  are  not  green,  as,  for  example,  the  " dodder"  and 
"indian  pipe."     But  these  plants  are  also  exceptional  in  other 
ways.     The  red  and  brown  sea  weeds  and  plants  like  the 
coleus  are  apparently  exceptions,  but  in  these  cases  the  green 
is  really  present,  though  masked  by  other  coloring  matters. 
Besides,  there  is  a  large  group  of  organisms  like  toadstools 
and  molds,   collectively  called  fungi,    which    are   not   green. 
These  organisms  are  grouped  by  the  biologist  with  the  plants, 
but  they  are  evidently  very  different  from  what  is  commonly 
meant  by  "  plants,"  and  for  the  present  we  may  leave  them 
out  of  consideration.     So  we  may  say  that,  with  some  excep- 
tions, those  organisms  which  are  commonly  called  plants,  are 
green.     Such  uniformity  of  color  is  not  found  among  animals 
and,  therefore,  it  is  worth  while  to  ask,  why  are  plants  so 
uniformly  green? 

19.  Most  plants  which  are  normally  green  lose  their  color 
when  grown  in  the  dark.     Thus  grass  growing  beneath  a  stone 
is  yellow;  celery  is  blanched  by  covering  it,  and  the  shoots  of 
potatoes  sprouting  in  a  dark  bin  have  no  trace  of  green. 

20.  Exposure  to  sunlight  soon  produces  the  familiar  green 
in  the  leaves  and  certain  parts  of  the  stem  of  such  etiolated 
plants.     Frequently  those  parts  of  a  plant  which  are  not  nor- 
mally green  become  so  on  exposure  to  sunlight.     This  occurs 
when,  for  example,  the  tubers  of  an  Irish  potato,  normally 
underground,  are  exposed  by  removal  of  the  soil.     The  same 
is  true  of  the  roots  of  many  plants. 

21.  Furthermore,  it  will  be  found  that  no  green  plants  will 
continue  to  grow  in  places  where  they  can  get  no  light;  while 
on  the  other  hand  fungous  growths  like  toadstools  flourish  in 


1 6  PLANTS 

cellars,  caves,  hollow  logs  and  similar  dark  situations  where 
green  plants  are  never  found. 

22.  There  seems,  therefore,  to  be  a  direct  relation  between 
the  green  color  of  plants  and  the  sunlight,  and  this  becomes 
still  more  evident  when  we  consider  the  distribution  of  the 
green  on  the  individual  plant.     In  the  smaller  herbaceous 
plants  the  green  may  be  found  in  all  parts  above  ground,  in 
stem  and  leaves  alike;  but  in  the  larger  perennial  growths,  like 
the  oak  tree,  the  green  is  found  only  in  the  leaves  and  twigs; 
possibly  also  on  the  surface  of  the  smaller  branches  and  between 
the  ridges  of  dead  bark  on  the  larger  limbs  and  the  trunk  itself. 
But  in  the  dark  central  parts  of  the  branches  and  trunk,  and 
beneath  the  thick  ridges  of  dead  bark   there  is  no  green.     It 
may  be  wanting  entirely  in  the  stem,  but,  with  the  rarest 
exceptions,  it  is  always  present  in  the  leaves. 

The  Leaves 

23.  In  connection  with  its  color  it  is  also  important  to  note 
the  form  of  the  leaf.     This  varies  through  an  infinite  variety 
of  patterns  from  circular  to  linear,  but  in  almost  every  case  it 
is  either  very  small,  or  else  very  thin  or  very  slender  in  propor- 
tion to  its  other  dimensions.     From  this  it  results  that  the 
surface  of  the  leaf  is  large  in  proportion  to  its  volume,  and  all 
of  its  substance  lies  near  the  surface,  that  is,  exposed  to  the 
light.     In  other  words,  the  form  of  the  leaf  is  such  as  to  give  a 
maximum  exposure  of  its  substance  to  the  light.     As  a  general 
characteristic  of  leaves  this  one  of  form  is  second  in  importance 
only  to  that  of  the  green  color. 

24.  A  typical  leaf  consists  of  three  parts:  (i)  a  broad,  thin 
portion — the  blade,  (2)  a  narrow  rounded  or  angular  stem — 
the  petiole,  and  (3)  at  the  base  of  the  petiole  a  pair  of  wing-like 
appendages — the  stipules.     Stipules  vary  greatly.     They  are 
usually  more  or  less  leaf-like,  but  may  be  reduced  to  mere 


LEAVES  17 

rudimentary  structures  or  may  even  be  entirely  wanting.  The 
leaf  may  also  be  without  a  petiole,  in  which  case  the  blade  is 
directly  connected  with  the  stem  or  branch  of  the  plant  and  is 
then  said  to  be  sessile. 

25.  The  petiole  is  stiff  enough  to  support  the  blade  and  yet 
is  flexible  and  elastic.     It  is  composed  largely  of  fibrous  woody 
tissue,  which  extends  on  from  the  petiole  into  the  blade,  where 
it  is  so  disposed  as  to  constitute  a  framework  upon  which  the 
more  delicate  tissues  of  the  blade  are  supported.     This  frame- 
work is  made  up  of  one  or  a  few  large  ribs  which  by  branching 
give  rise  to  numerous  smaller  veins  and  veinlets. 

26.  The  veins  may  be  arranged  in  one  of  two  ways;  they 
either  lie  parallel  with  one  another  and  extend  from  the  base 
of  the  blade  to  its  tip  or  from  the  single  large  midrib  to  the 
edge  of  the  blade,  or  else  they  unite  with  each  other  in  such  a 
way  as  to  form  a  network.    Leaves  having  the  former  arrange- 
ment of  the  veins  are  said  to  be  parallel  veined,  while  those  pre- 
senting the  latter  condition  are  termed  netted  veined.     Netted 
veined  leaves  may  further  be  characterized  as  feather  veined 
if  there  is  only  a  single  large  midrib  from  which  the  principal 
veins  branch  on  either  side,  or  palmately  veined  when  there  are 
several  ribs  spreading  in  a  fan  shaped  order  from  the  base  of 
the  blade. 

27.  The  upper  and  lower  surfaces  of  the  leaf  blade  are  formed 
by  thin,  transparent,  but  rather  tough,  membranes,  the  epider- 
mis, which  may  be  stripped  off.     Between  the  two  layers  of 
epidermis  lies  the  green  mesophyll,  which  next  the  upper  epider- 
mis forms  a  rather  firm  tissue,  but  on  the  lower  side  is  more 
spongy  in  texture. 

28.  In  outline  the  leaf  blade  is  extremely  variable.     All  forms 
from  circular  to  narrow  ribbon-like  or  even  thread-like  are  met 
with,  and  the  margin  varies  from  a  continuous  line  or  unbroken 
curve  to  conditions  which  may  be  described  as  toothed,  lobed, 
cleft  or  divided,  as  the  case  may  be.     A  divided  leaf  is  one  in 


1 8  PLANTS 

which  the  indentations  of  the  margin  extend  completely  to  the 
midrib,  thus  producing  a  double  series  of  leaflets  ranged  along 
a  common  midrib. 

29.  When  the  divisions  of  the  blade  are  all  distinct  so  that 
each  resembles  a  miniature  leaf,  the  leaf  is  said  to  be  compound. 
The  divisions  are  then  called  leaflets  and  the  common  leaf- 
stalk is  the  rachis. 

Phyllotaxy 

30.  Leaves  are  usually  arranged  on  the  stem  in  a  definite 
order.     On  a  vertical  shoot  there  are  two  or  more  vertical  ranks 
of  leaves.     When  there  are  two  leaves  at  the  same  level  they 
are  opposite,  and  each  pair  crosses  the  pair  above  or  below  at 
right  angles,  making  four  vertical  ranks  of  leaves.     Sometimes 
there  are  three  or  more  leaves  at  the  same  level,  forming  a 
whorl.     If  there  is  only  one  leaf  at  each  level  the  leaves  are  said 
to  alternate.     In  this  case  every  2nd,  3rd,  5th,  8th  or  i3th, 
etc.,  leaf,  as  the  case  may  be,  is  in  the  same  rank  and  there  will 
be  2,  3,  5,  8,  or  13,  etc.,  ranks  respectively.     This  order  is  in 
reality  a  spiral  one,  for  if  a  line  is  drawn  from  one  leaf  to  the  next 
higher  one  in  the  nearest  direction  and  continued  in  this  way  it 
will  describe  a  spiral  around  the  stem.     This  methodical  ar- 
rangement of  the  leaves  evidently  gives  each  leaf  a  maximum 
of  elbow  room  with  respect  to  its  fellows,  and  tends  to  equalize 
the  conditions  of  light  and  shade. 

31.  The  number  of  leaves  which  may  receive  sufficient  light 
exposure  on  a  stem  of  a  given  length  depends  on  (i)  the  size  of 
the  leaf — a  few  large  ones  will  shade  each  other  as  much  as  a 
large  number  of  small  ones,  (2)  the  shape  of  the  blade — long, 
narrow  leaves  or  finely  divided  ones  may  be  set  more  closely 
than  broad  and  entire  leaves,  (3)  the  length  of  the  petiole- 
other  things  being  equal  a  long  petiole  will  give  the  leaves 
more  room  than  a  short  one  and  consequently  long  petioles 


THE   STEM  IQ 

are  usually  associated  with  broad  leaves.  From  this  it  follows 
that  there  is  a  correlation  between  the  number  of  ranks  of 
leaves  on  the  stem  and  the  distance  between  leaf  levels  on  the 
one  hand,  and  the  form  and  size  of  the  leaf  on  the  other. 
Plants  which  normally  grow  in  tussocks,  i.  e.,  many  stems  in  a 
cluster,  form  a  natural  exception  to  the  above  rule,  for  in  this 
case  the  crowding  of  the  stems  reduces  the  number  of  leaves 
possible  on  each  stem. 

32.  On  horizontal  branches  the  leaves  are  attached  to  the 
stem  in  precisely  the  same  order  as  on  vertical  stems,  but  the 
blades  of  the  leaves  are  in  many  cases  brought  round  into  the 
horizontal  plane  by  a  twisting  and  bending  of  the  petiole. 

33.  Still  other  devices  are  employed  for  securing  equal  and 
sufficient  illumination  of  the  leaves,  but  whatever  the  means 
employed  all  tend  toward  the  same  result,  viz.,  a  maximum 
exposure  of  green  tissue  to  the  light. 

The  Stem 

34.  From  what  has  gone  before,  it  is  evident  that  one  of 
the  functions  of  the  stem  of  the  plant  is  to  hold  up  the  leaves 
to  the  light.     This  function  may  be  performed  in  various  ways, 
and  much  of  the  character  of  the  stem  and  its  branches  depends 
upon  how  this  function  is  performed.     Among  the  low  herba- 
ceous forms  the  adaptation  of  the  stem  to  this  function  is  simple 
enough  and  nothing  further  need  be  said  here,  except  to  note 
that  where  the  plants  are  crowded  the  stems  are  usually  less 
branched,   and  more  slender   than  where   they  grow  singly. 
This  is  also  true  of  trees,  and  the  cause  of  it  may  be  discovered 
by  the  comparison  of  a  few  examples. 

35.  A  tree  growing  in  an  open  space  tends  to  have  a  relatively 
short  and  thick  trunk,  with  large,  spreading  branches  near  the 
ground.     One  growing  close  by  the  side  of  another  in  an  open 
field  will  have  the  large  branches  only  on  the  side  away  from 


2O 


PLANTS 


the  neighboring  tree.  If  the  tree  is  closely  surrounded  by  others 
of  approximately  the  same  age,  as  in  a  forest,  the  trunk  will  be 
taller  in  proportion  to  its  diameter  and  there  will  be  no  large 
limbs  near  the  ground.  These  facts  show  clearly  that  the  larger 
branches  develop  only  where  they  can  reach  the  light,  whereas 


FIG.  3. — Three  oak  trees  'in  a  group,  showing  the  effect  of  one  tree  on 
another  with  regard  to  the  development  of  the  branches.  In  the  middle  tree 
the  branches  extend  toward,  and  away  from,  the  observer. 

those  twigs  which  appear  from  time  to  time  in  shaded  situ- 
ations fail  to  develop  into  large  branches  because  of  the  lack  of 
light,  and,  after  a  few  seasons'  struggle  against  adverse  circum- 
stances, ultimately  die  and  fall  away. 

36.  The  stem  of  erect  plants  is  usually  a  cylinder  of  woody 


THE   ROOTS  21 

tissue.  Woody  tissue  possesses  in  a  large  degree  both  rigidity 
and  elasticity,  while  the  cylindrical  form  is  of  all  forms  the  one 
giving  greatest  rigidity.  The  stem  cylinder  is  often  hollow, 
but  usually  the  axis  is  occupied  by  a  core  of  spongy  tissue — the 
pith.  This  by  itself  would  be  of  little  value  as  a  supporting 
structure  and  yet  in  young  shoots  it  probably  adds  greatly  to 
the  strength  of  the  stem  by  preventing  buckling  of  the  cylinder. 
In  far  the  greater  number  of  the  plants  there  is  also  a  cylinder 
of  bark  which  surrounds  the  woody  part.  This  bark  has  a 
double  function.  It  adds  greatly  to  the  elasticity  of  the  stem 
through  the  layer  of  fibers — the  bast — which  lies  directly  over 
the  wood  and  which  possesses  great  tensile  strength.  The 
other  function  of  the  bark — that  of  protection — is  subserved 
by  its  outer  layers  which  consist  either  of  a  smooth  and  tough 
epidermis  or  thick  layers  of  corky  tissue,  both  of  which  are 
highly  resistant  to  mechanical  injury. 

The  Roots 

37.  The  stem  of  the  plant  is  firmly  anchored  in  the  soil  by 
the  roots.  Continuing  downward  from  the  base  of  the  stem 
there  is  often  a  short,  rapidly  tapering  tap  root,  while  other 
and  longer  roots  pass  out  radially  and  usually  at  a  small  angle 
downward.  These  master  roots  branch  repeatedly,  giving  rise 
ultimately  to  a  vast  number  of  minute  rootlets  which  interlace 
and  penetrate  the  soil  in  all  directions  and  through  a  space  of 
considerable  radius.  The  tap  root  is  often  insignificant  and 
the  plant  is  held  erect  by  the  combined  bracing  and  guying 
action  of  the  lateral  roots.  Only  the  larger  roots  where  they 
unite  with  the  stem  possess  any  great  degree  of  rigidity.  The 
more  remote  parts  of  the  root  system  have  little  rigidity  or 
elasticity  in  comparison  with  the  stem  and  branches.  They 
are,  on  the  other  hand,  quite  flexible  and  tough  and  capable  of 
resisting  a  considerable  pull  longitudinally. 


22  PLANTS 

38.  In  many  cases  there  are  a  large  number  of  fibrous  roots 
which  spring  directly  from  the  base  of  the  stem  instead  of  a 
few  larger  roots.     Such  fibrous  root  systems  occur  principally 
on  low  herbaceous  plants  which  do  not  require  an  especially 
strong  supporting  system. 

39.  The  structure  of  the  root,  in  the  main,  resembles  that  of 
the  stem  in  that  there  is  a  central  woody  axis  and  an  outer 
bark;  but  there  is  no  pithy  core.     The  woody  portion  is  not  as 
firm  as  that  of  the  stem,  but  it  is  of  ten  very  tough.     The  outer, 
dead,  protective  layer  of  the  bark  of  roots  is  also  relatively 
thin. 

40.  In  the  preceding  paragraphs  some  of  the  most  evident 
characteristics  of  ordinary  plants  have  been  noted  for   the 
purpose  of  showing  that  the  form  and  structure  of  plants  is  in 
every  case  an  adaptation  to  a  certain  end,  and  that  even  the 
peculiarities  of  each  kind  of  plant  are  special  adaptations  to 
special  ends.     In  continuing  our  studies  we  shall  constantly 
keep  in  mind  this  idea  of  adaptation  to  functions,  that  is  to 
say,  at  every  point  we  shall  seek  to  answer  the  question  why? 

41.  In  the  following  paragraphs  we  shall  take  up  for  con- 
sideration, in  turn,  the  seed,  the  developing  seedling  and  the 
mature  plant,  studying  in  each  case  the  structure  and  functions 
of  the  principal  sets  of  organs,  so  that  at  the  end  we  may  have 
a  fairly  comprehensive  idea  of  the  life-history  of  a  plant. 

Seeds 

42.  Seeds  present  a  remarkable  diversity  of  form  and  struc- 
ture, but  there  are  usually  two  distinct  sets  of  organs  to  be 
recognized.     The  first  of  these  are  the  seed  coats,  evidently 
organs  of  protection,  often  consisting  of  an  outer  firm  layer, 
the  testa,  and  an  inner  membraneous  layer,  the  tegmen.     How- 
ever, the  seed  coats  may  be  variously  modified  and  cannot 
be  generally  characterized. 


GERMINATION  23 

43.  The  other  set  of  organs  is  the  essential  part  of  the  seed 
and  constitutes  the  germ  or  embryo.     In  the  largest  of  the  three 
grand  divisions  of  seed-bearing  plants,  the  Dicotyledons,  the 
embryo  consists  usually  of  two  symmetrical  parts,  the  cotyle- 
dons, which  are  connected  by  a  third  part — the  caulicle.     At 
the  end  of  the  caulicle  between  the  cotyledons  there  may  also 
be  a  minute  structure,  the  plumule,  which  when  well  developed 
shows  clearly  the  outlines  of  one  or  more  leaves  in  miniature. 

44.  In  another  division  of  the  seed-bearing  plants  there  is 
only  one  cotyledon,  and  hence  the  name  applied  to  the  group 
is  Monocotyledons. 

45.  Besides  the  embryo  there  is  frequently  contained  within 
the  seed  coat  a  mass  of  food  material  for  the  use  of  the  develop- 
ing embryo.     This  food  material,  called  endosperm  or  peri- 
sperm,  depending  on  the  position  it  occupies  in  the  seed,  may 
consist  either  of  starch,  proteid,  oil,  or  cellulose,  or  a  combina- 
tion of  two  or  more  of  these  food  principles. 

Germination 

46.  The  conditions  necessary  for  the  germination  of  seeds 
are:     First,  a  favorable  temperature  which  might  be  designated 
as  warmth.     The  range  and  limits  of  this  favorable  temperature 
are  not  sharply  denned  and  may  vary  with  the  kind  of  seed. 
Cold,  a  temperature  below  the  limit  within  which  germination 
takes  place,  indefinitely  retards  development,  though  it  does 
not  necessarily  destroy  the  vitality  of  the  germ  if  the  seeds  are 
dry,  while  on  the  other  hand,  any  considerable  increase  of 
temperature  above  that  of  germination  destroys  all  power  of 
further  development. 

47.  A  second  condition  of  germination  is  moisture.     This 
softens  the  seed  coats,  thus  permitting  the  embryo  to  expand, 
and  also  supplies  the  water  which  is  everywhere  necessary  to 
growth. 


24  PLANTS 

48.  That  oxygen  is  necessary  to  germination  may  be  demon- 
strated by  experiment  either  by  placing  the  seeds  of  aquatic 
plants  in  water  from  which  the  air  has  previously  been  expelled 
by  boiling,  or  by  placing  seeds  in  a  vessel  containing  an  atmos- 
phere deprived  of  its  oxygen. 

49.  Under  the  conditions  just  enumerated  the  embryo  swells 
through  the  absorption  of  water,  the  seed  coats  burst  and  the 
caulicle  grows  out  and  down  into  the  soil,  the  terminal  part  of 
it  going  to  form  the  primary  root.     It  will  be  noted  here  that 
the  direction  of  growth,  not  only  of  the  root,  but  also  of  the  other 
growing  parts  of  the  plant,  is  very  definite  and  that  the  deter- 
mining cause  of  it  must  be  sought  in  some  external  agency. 
By  suitably  conducted  experiments  we  find  that  gravity  acts 
upon  the  primary  root  as  a  stimulus,  in  response  to  which  it 
grows  downward.     This  response  is  known  as  positive  geotrop- 
ism.     If  the  influence  of  gravity  be  eliminated  the  root  will  turn 
toward  the  source  of  moisture — this  is  positive  hydrotropism. 

50.  The  plumule  also  responds  to  external  stimuli,  but  in 
a  different  way.     It  turns  away  from  the  earth,  being  negatively 
geotropic,  and  grows  toward  the  light — positively  heliotropic. 
(The  student  should  note  that  unless  light  actually  impinges 
on  the  seedling  it  can  have  no  influence  in  determining  the 
direction  of  growth.     Hence,  if  the  seed  is  growing  in  the 
dark  the  direction  of  growth  must  be  determined  by  some 
stimulus  other  than  light.     In  this  connection  analyze  care- 
fully the  results  of  experiments  i  and  2  under  paragraph  30, 
and  31,  page  10.) 

The  Seedling 

51.  When  the  primary  root  has  penetrated  the  soil  some 
distance,  lateral  branches  begin  to  appear  on  all  sides  of  it 
at  some  distance  above  its  tip.     These  branches  are  not  posi- 
tively geotropic,  since  they  grow  in  an  almost  horizontal  direc- 
tion— diageotropism — with  perhaps  a  slight  tendency  down- 


THE    SEEDLING  25 

ward.  With  the  appearance  of  the  lateral  rootlets  there  can, 
of  course,  be  no  further  elongation  of  that  part  of  the  radicle 
or  tap  root  from  which  they  spring,  since  this  would  result  only 
in  the  destruction  of  the  branch  roots  or  a  doubling  of  the  tap 
root.  Observation  of  a  marked  primary  root  shows,  in  fact, 
that  elongation  takes  place  only  near  the  tip.  The  subsequent 
development  of  the  root  system  is  simple  enough.  The  main 
branches  increase  in  diameter,  and,  as  they  push  out  farther 
into  the  soil,  give  off  numerous  smaller  branches.  Successive 
branching  in  this  way  finally  produces  a  system  which  ends 
in  innumerable  minute  rootlets. 

52.  With  the  development  of  the  lateral  roots  the  seedling 
becomes   firmly   anchored  in   the  soil.     This  is   a  necessary 
preliminary  in  many  plants  to  the  first  steps  in  the  development 
of  the  stem.     In  some  cases  the  conical  plumule  pushes  upward 
through  the  soil  as  the  radicle  grows  downward,  without  moving 
the  cotyledons.     In  other  cases  the  cotyledons  are  forced  up 
through  the  soil  before  the  plumule  has  undergone  any  con- 
siderable development.     This  is  accomplished  by  the  elongation 
of  that  part  of  the  seedling — called  the  hypocotyl — which  lies 
between  the  cotyledons  and  the  first  lateral  roots.     With  one 
end  fixed  by  its   root  anchorage,   the  elongating  hypocotyl 
carries  the  cotyledons  upward  in  the  direction  of  least  soil 
resistance.     During  this  process  the  seed  coats  are"  stripped  off, 
and,  as  soon   as   the  cotyledons   appear   above  ground,   the 
plumule  is  free  to  continue  its  development. 

53.  The  several  functions  of  the  cotyledons  now  become 
evident.     In  those  cases  where  they  remain  in  the  soil  they  are 
either  greatly  swollen  by  the  reserve  food  contained  in  the  cotyle- 
dons themselves,  or  else  they  are  embedded  in  a  large  store  of 
endosperm  or  perisperm.     In  either  case  they  nourish  the  grow- 
ing embryo  from  the  stored  food  supply.     The  plumule  of  such 
seeds  is  a  conical  shaft,  well  adapted  to  bore  its  way  through 
the  ground. 


26  PLANTS 

54.  In  those  cases  in  which  the  cotyledons  appear  above  the 
ground  they  serve  to  protect  the  delicate  plumule  while  the 
vigorous  hypocotyl  is  pushing  it  up  through  the  earth.     If  the 
cotyledons  in  this  case  are  greatly  thickened  they  are  likely  to 
become  shriveled  as  they  give  up  their  food  to  the  seedling, 
and  finally  they  may  fall  off.     Again  they  may  become  green 
and  serve  for  a  time  the  functions  of  leaves.     Very  often  they 
are  clearly  leaflike  at  the  beginning  and  remain  for  some  time 
as  the  first  pair  of  leaves.     In  every  case  the  cotyledons  nourish 
the  seedling,  either  from  endosperm  or  perisperm,  or  from  the 
food  contained  within  their  own  tissues,  until  green  leaves  are 
developed  by  transformation  of  the  cotyledons  themselves  or 
by  the  development  of  the  first  leaves  by  the  plumule. 

55.  The  plumule  is  the  last  of  the  embryonic  parts  to  begin 
its   development.     From   it   arise   practically   all   the   above- 
ground  parts  of  the  plant,  i.  e.,  stem  and  leaves.     In  the  embryo 
it  is  essentially  a  bud,  and,  as  it  develops,  one  segment  of  the 
stem  after  another  appears  and  leaf  after  leaf  unfolds  until  we 
have  the  fully  formed  plant.     The  region  of  development,  i.  e., 
the  formation  of  new  parts,  in  the  plumule  is  at  the  apex  of 
its  axis  in  the  center  of  the  bud,  but  after  the  parts  have  been 
formed  and  unfolded  they  continue  to  expand  for  some  time. 
From  this  primary  bud,  which  is  first  called  the  plumule,  but 
later  on  is  known  as  the  terminal  bud,  is  developed,  directly, 
the  main  axis  or  stem  of  the  plant  with  its  leaves. 

56.  Secondary  axes,  or  branches,  are  developed  from  buds 
(axillary  buds)  which  appear  in  the  angles  (the  axil)  between 
the  leaves  and  the  stem.     In  the  case  of  perennial  plants  the 
leaves  which  form  last,  but  do  not  unfold  in  the  fall,  and  which 
are  the  first  to  unfold  in  the  following  spring,  are  scale-like  in 
form  and  serve  to  protect  the  tender  parts  which  they  enfold, 
from  the  winter  weather. 

57.  In  some  cases  accessory  buds  occur  above  or  on  either 
side  of  the  axillary  bud,  and  adventitious  buds  may  occur  on 


THE    MATURE   PLANT  27 

any  part  of  the  stem.  In  case  a  terminal  bud  is  destroyed,  and 
also  under  certain  other  conditions,  the  development  of  the 
main  axis  may  be  continued  by  an  axillary  bud.  Also,  if  an 
axillary  bud  is  destroyed  its  functions  may  be  taken  up  by  ac- 
cessory or  adventitious  buds. 

58.  Since  branches  normally  develop  from  axillary  buds,  it 
follows  that  branches  are  arranged  on  the  stem  in  conformity 
with  the  law  which  governs  the  arrangement  of  the  leaves  on 
the  stem. 

59.  The  terminal  bud,  because  of  its  favorable  position  with 
respect  to  light  exposure,  and  also  possibly  for  other  causes,  is 
usually  stronger  than  lateral  buds,  and  therefore  the  main  axis 
develops  more  rapidly  than  the  branches.     Many  lateral  buds, 
on  the  other  hand,  are  in  such  unfavorable  positions  that  even 
after  having  developed  to  some  extent  they  are  " choked"  and 
the  twig  dies  and  falls  away.     Still  others  never  develop  at  all. 
Thus  it  results  that  while  the  position  of  a  branch  on  the  stem 
is  governed  by  the  law  of  leaf  arrangement,  yet,  because  of  the 
large  number  of  buds  that  do  not  develop  and  of  others  that  are 
choked  out,  the  regularity  of  arrangement  is  seldom  evident  in 
the  case  of  branches. 


The  Mature  Plant 

60.  At  the  end  of  the  growing  season  the  foliage  leaves  of 
deciduous  perennials  fall  off,  leaving  a  scar  on  the  twig.  The 
bud  scale-leaves  fall  away  on  the  unfolding  of  the  bud  and  also 
leave  scars,  which,  however,  are  so  crowded,  because  of  the 
slight  elongation  of  the  axis,  that  they  frequently  form  a  con- 
tinuous ring  around  the  stem.  The  scale-leaf  scar  can  also  be 
distinguished  from  the  foliage-leaf  scar  by  its  form.  The 
position  of  the  scale-leaf  scars  indicates  the  beginning  of  the 
year's  growth,  consequently  the  age  of  a  twig  may  be  deter- 
mined by  counting  the  successive  rings  of  scale  scars.  Other 


28 


PLANTS 


characters,  such  as  the  color  and  texture  of  the  bark  and  th< 
succession  of  branches,  will  also  serve  to  determine  the  age  o 
any  particular  section  of  the  branch. 

61.  If  we  cut  across  a  twig  of  one  year's  growth,  we  find  tha 
it  consists  of  three  parts,  an  outer  bark  which  may  be  peelec 
off,  a  central  core  of  soft  tissue — the  pith — and  between  them 


m 


FIG.  4. — Photomicrograph  of  a  cross  section  of  oak  wood  showing  one  year' 
growth.  E,  Early  growth;  L,  late  growth;  m  and  n,  large  and  small  medullary 
rays.  (From  Stevens.) 


a  firmer  cylinder,  the  wood.  The  outer  surface  of  the  bark  i 
smooth  and  rather  tender,  and  covers  a  layer  containing  more 
or  less  green  substance.  The  inner  layers,  those  which  are  nex 
the  wood,  are  hard  and  consist  largely  of  very  tough  fibers 
The  pith  is  soft  and  spongy  in  texture  and  contains  no  fibers 
The  wood  is  also  fibrous,  since  it  can  be  split  lengthwise  of  th( 


COMPOSITION   OF   PLANTS  2Q 

stem,  but  it  is  more  compact  than  the  fibrous  tissue  of  the 
bark  and  cannot  be  as  readily  separated  into  strands. 

62.  If  twigs  two  and  three  years  old  are  cut  across  we  find 
that  there  are  differences  besides  merely  that  of  thickness.     In 
the  older  stems  the  surface  of  the  bark  has  changed  color, 
become  firmer  and  also  perhaps  rougher.     There  is  less,  if  any, 
evidence  of  chlorophyll,  and  the  bark  is  thicker.     The  pith 
shows  little  change,  but  the  woody  cylinder  is  about  twice  or 
three  times  as  thick  as  before  and  is  divided  by  concentric 
circles  into  annual  rings  of  growth.     Crossing  these  circles  of 
growth  at  right  angles  are  narrow  radial  lines  of  pith  which 
connect  the  central  pith  core  with  the  bark.     These  are  the 
medullary  rays. 

Composition  of  Plants 

63.  It  is  evident  that  water  constitutes  a  very  large  per 
cent,  of  the  substance  of  plants.     If  a  portion  of  plant  tissue  be 
weighed  and  then  subjected  to  a  moderately  high  temperature 
until  it  is  thoroughly  dried  and  then  weighed  again,  it  will  be 
found  to  have  lost  from  50  to  95  per  cent,  of  its  weight.     In  suc- 
culent herbs  the  percentage  of  water  is  very  great,  while  in 
woody  tissues  it  is  much  less.     A  moment's  thought  will  show 
that  the  water  contained  in  plants  must  be  absorbed  chiefly  by 
the  roots,  for  plants  may  grow  and  flourish  even  though  water 
never  falls  upon  the  stem  and  leaves. 

64.  If  after  thoroughly  drying  vegetable  tissue  the  tempera- 
ture be  increased  to  just  short  of  the  point  of  ignition  the  tissue 
becomes  black  and  there  finally  remains  only  a  mass  of  charcoal 
(carbon),  equal  in  weight  to  about  25  per  cent,  of  the  dried  mass. 
During  the  process  of  charring  various  vapors  and  gases  are 
driven  off;  among  others  are  the  vapor  of  water  (H2O),  carbon 
dioxide  (C02),  carbon  monoxide  (CO),  marsh  gas  (CH4)    and 
other  hydro-carbons.     After  complete  ignition  of  the  charcoal 


PLANTS 


there  is  left  a  small  residue  of  ash,  amounting  to  about  5  per 
cent,  or  less,  of  the  dried  substance. 

65.  The  ash  consists  chiefly  of  the  following  mineral  sub- 
stances,   viz.:  Potash,    soda,    lime,    magnesia,    phosphorous, 


FIG.  5. — Experiment  to  determine  the  composition  of  vegetable  tissue.  A 
simple  apparatus,  consisting  of  test-tubes,  glass  tubing  and  cork  stoppers,  is 
fitted  up  as  shown  in  the  figure.  The  tube  A  should  be  of  hard  glass.  A  piece 
of  dry  wood  (W)  is  then  heated  over  a  burner,  at  first  gently,  then  more  vigorously, 
until  it  is  reduced  to  charcoal.  At  first  water  is  driven  off  and  condenses  in  the 
cold  tube  CB).  Then  other  volatile  substances  pass  over,  some  of  which  con- 
dense in  B  and  others  escape  at  C.  The  latter  may  be  tested  for  H2O  and  CO2. 
The  jet  escaping  at  C  may  then  be  ignited  and  the  flame  tested  for  H2O  and 
CO2.  The  liquid  which  has  collected  in  the  tube  B,  is  wood  vinegar  and  contains 
water,  acetic  acid,  wood  alcohol  and  tar.  Test  with  litmus  paper  for  acid. 
Then  heat  until  it  boils,  when  a  blue  flame  may  be  obtained  at  C.  This  is  due 
to  the  volatilized  alcohol. 

sulphur,  silica,  chlorine,  and  manganese,  which  are  evidently 
derived  from  the  soil  and  must  therefore  have  been  absorbed 
by  the  roots. 

66.  Plants  will  thrive  if  the  water  supplied  to  the  roots  con- 
tains only  the  above  minerals  and  a  trace  of  iron.     But  the 


COMPOSITION   OF   PLANTS  31 

largest  constituents  of  the  plant  are  carbon,  about  45  per  cent,  or 
more,  and  oxygen,  about  45  per  cent,  or  less.  Since  the  carbon 
is  not  necessarily  present  in  the  water  it  must  be  derived  from 
some  other  source.  Carbon  is  present  in  the  atmosphere  in 
small  quantities,  combined  with  oxygen  in  the  form  of  carbon- 
dioxide  (CO2),  and  in  the  absence  of  this  gas  the  plant  will  not 


FIG.  6. — The  preceding  experiment  may  be  performed  more  satisfactorily  by 
substituting  an  iron  capsule  for  the  hard  glass  test-tube  and  connecting  the 
delivery  tube  with  a  Leibig  condenser.  Such  an  arrangement  is  represented  in 
Fig.  6. 

thrive.  Consequently  we  must  assume  that  the  carbon  is 
absorbed  from  the  atmosphere  by  the  stem  and  leaves.  This 
conclusion  may  be  verified  by  experiment. 

67.  The  oxygen  taken  up  by  the  plant  may  be,  and  as  a 
matter  of  fact  is,  taken  up  in  part  as  free  oxygen  from  the 
atmosphere,  in  part  in  combination  with  carbon  as  CC>2,  and  in 
part  in  combination  with  other  elements  absorbed  by  the  roots. 


PLANTS 


Structure  and  Function  of  the  Roots 

68.  The  mechanism  of  water  absorption  by  the  roots  may  be 
discovered  by  the  study  of  cross  sections  of  the  smaller  root- 
lets. Such  a  section  taken  several  centimeters  from  the  tip, 
i.  e.,  through  the  region  covered  by  root  hairs,  presents  three 
well  marked  kinds  of  tissues;  viz.,  (i)  a  general  ground  tissue 
made  up  of  rounded  or  polygonal  cells  with  thin  walls,  (2) 
larger  circular  structures  grouped  around  the  axis  of  the  root, 

which  are  longitudinal  vessels 
in  cross  section,  and  (3)  root 
hairs,  which  are  tubular  ex- 
pansions of  some  of  the  thin 
walled  cells  of  the  surface 
layer.  The  vessels  may  usu- 
ally be  seen  by  the  unaided 
eye,  especially  in  the  larger 
roots.  The  root  hairs  are 
very  conspicuous  and,  when 
growing  in  a  moist  atmos- 
phere, stand  up  rigidly  from 
FIG.  7.— Cross  section  of  a  young  root,  the  surface  of  the  root  as 

slender     cylindrical     bodies 

several  millimeters  in  length.  If  they  are  exposed  for  a  few 
minutes  to  the  dry  air  they  soon  become  limp,  topple  over  and 
shrivel.  This  fact  shows  that  the  watery  content  of  the  hair 
is  rapidly  extracted  by  evaporation  from  the  surface,  and  that, 
therefore,  the  cell  wall  of  the  hair  is  highly  pervious  to  water. 
69.  By  cutting  off  the  stem  of  a  growing  plant  near  the 
ground  and  connecting  a  glass  tube  with  the  stump  it  may  be 
shown  that  the  roots  have  the  power,  not  only  of  absorbing 
moisture  from  the  soil,  but  also  of  driving  the  sap  up  into  the 
stem  under  considerable  pressure.  In  all  probability  the 
force  chiefly  responsible  for  this  root  pressure  is  the  osmotic 


STRUCTURE   AND  FUNCTION   OF  ROOTS 


33 


FIG.  8. — Cross  section  of  rootlet  in  the  region 
of  the  root  hairs.     (From  Stevens.) 


action  which  takes  place  between  the  contents  of  the  root  hairs 

and  the  soil  water  through  the  cell  walls  of  the  root  hairs;  these 

cell    walls    being    admirably    adapted    to    serve   as    osmotic 

membranes. 

70.    Soil    water    holds 

various    mineral    salts   in 

solution  in  small  quanti- 

ties.    These  are  absorbed 

with    the    water    and 

furnish   the  mineral  con- 

stituents of  the  ash.     At 

the   same   time    carbonic 

acid  passes  out  from  the 

root  hairs  into  the  soil  and 

by  its  solvent  action  helps  to   break  up  the  mineral  constitu- 

ents of  the  soil,  thus  serving  at  once  to  disintegrate  the  rocks 

and  also  increase  the  quantity  of 
mineral  salts  contained  in  the  soil  water. 
7  1  .  The  fluids  absorbed  by  the  root 
hairs  may  then  also  be  transferred 
from  cell  to  cell  by  osmotic  action  and 
thus  finally  reach  the  tubular  vessels 
which  lie  near  the  axis  of  the  root. 
These  vessels  form  a  conducting  tissue 
through  which  the  fluids  may  travel 
freely,  propelled  by  the  osmotic  force 

between  the  dotted  lines  is    of  the  thousands  of  root  hairs  on  the 

shown  on  a  larger   scale  in  .    ,  .  ,  , 

the  next  figure.  periphery  of  the  root. 


FIG.  9.—  Diagram  to  show 


Structure  and  Function  of  the  Stem 

72.  The  structure  of  the  stem  differs  somewhat  in  its  sig- 
nificant features  from  that  of  the  root.  In  a  cross  section  of 
a  young  stem  we  find,  as  in  the  root,  a  ground  tissue  of  thin 

3 


34 


PLANTS 


walled  spherical  or  polygonal  cells.  Such  tissue  is  generally 
termed  parenchyma.  In  this  case  it  occupies  the  axis  of  the 
stem  and  forms  the  pith.  There  are  also  radial  extensions 
of  the  parenchyma  from  the  pith  toward  the  surface  of  the  stem. 
The  disposition  of  the  parenchyma  in  a  cross  section  might 


Epidermis 
Schlerenchyma 

Parenchyma 
Bast 


Cambium 


Wood 


Parenchyma 


FIG.  10. — Cross  section  of  a  typical  dicotyledon  stem  from  the  pith  to  the 
epidermis  and  comprising  one  vascular  bundle.     See  preceding  figure. 

therefore  be  likened  to  the  hub  and  spokes  of  a  wheel.  Ir 
the  position  corresponding  to  the  felloes  of  the  wheel  there  is 
also  more  or  less  parenchyma. 

73.  The  tire  of  the  wheel  is  represented  by  a  single  layei 
of  brick-shaped  cells  whose  outer  walls  are  thickened  and  forn 


STRUCTURE    OF   THE    STEM 


35 


a  continuous  layer  of  smooth,  tough  and  impervious  cuticula. 
This  layer  of  cells  is  the  epidermis.  It  is  sufficiently  elastic 
to  allow  considerable  expansion  with  the  growth  of  the  stem, 
but  it  may  finally  be  ruptured  and  scale  off,  leaving  the  under 
parts  exposed. 

74.  The  spaces  between  the  spokes  of  the  parenchyma  wheel 
are  occupied  by  a  system  of  fibers  and  vessels  known  as  the 
vascular  bundles.  The  fibers  are  usually  of  two  distinct 
types,  one,  known  as  bast,  is  found  nearer  the  surface  of  the 
stem,  while  the  other  is  the  chief  element  of  the  wood  and  lies 


ABC 

FIG.  n. — Diagrams  representing  the  structural  elements  of  the  vascular 
bundles.  A,  A  fibre  of  wood,  or  bast;  B,  one  end  of  a  tracheid,  showing  spiral 
markings;  C,  part  of  a  trachea,  or  true  vessel,  with  pitted  markings  and  the 
remnant  of  the  dividing  wall  which  originally  separated  two  of  the  cells  which 
helped  form  the  vessel;  D,  part  of  a  sieve  tube  with  the  perforated  cross  wall 
(sieve). 

nearer  the  pith.  The  bast  fibers  consist  of  greatly  elongated 
cells  with  extremely  thick  walls.  The  fibers  of  the  wood  are 
similar  but  the  walls  are  not  so  thick.  The  vessels  are  of  several 
kinds;  first,  tracheides,  consisting  of  single  elongated  cells 
whose  walls  are  unbroken,  but  variously  thickened  in  limited 
areas,  forming  rings,  spirals,  annular  pits,  etc. ;  second,  the  sieve 
vessels,  formed  by  rows  of  elongated  cells  placed  end  to  end 
with  the  dividing  walls  perforated  by  pores  forming  a  sieve; 


PLANTS 


•    >•    a    o    "  %          * 

3  £•&*>•  ti     i    ,5  *  ••      " ,• 

*-     C  >»    °          ^        "^o*"1  3 


FIG.  12. — A  diagram  to  show  the  character  of  the  tissues  and  their  disposition 
in  a  young  stem  of  the  typical  dicotyledon  type.     (From  Stevens.) 


STRUCTURE    OF   THE   STEM 


37 


FIG.  13.— Diagram  similar  to  the  preceding  but  representing  a  later  stage  and 
showing  the  tissues  formed  by  the  cambium.     (From  Stevens.) 


38  PLANTS 

third,  the  true  vessels,  which  originate  from  rows  of  cells  whose 
dividing  walls  disappear,  leaving  a  continuous  passage  from  cell 
to  cell.  The  walls  of  the  true  vessels  are  also  thickened  in 
spiral  lines  and  otherwise  as  in  the  tracheides.  The  tracheides 
and  vessels  lie  on  the  side  of  the  vascular  bundle  next  the  pith, 
while  the  bast  and  sieve  vessels  lie  next  the  surface  of  the  stem. 

75.  The  bast  and  wood  portions  of  the  vascular  bundles  are 
separated  by  a  zone  of  very  thin-walled  cells.     This  is  the 
cambium,  the  region  in  which  the  new  cells  are  formed  and  added 
to  the  tissues  on  either  side,  increasing  the  thickness  of  the  bark 
on  one  side  and  adding  to  the  wood  on  the  other.     The  delicate 
cambium  is  readily  torn  and  forms  the  line  along  which  the  bark 
separates  from  the  wood. 

76.  The  woody  portions  of  the  vascular  bundles  are  arranged 
side  by  side  around  the  pithy  axis,  thus  forming  the  cylinder 
alluded  to  above  (paragraph  36). 

77.  By  experiment  it  may  readily  be  determined  that  the 
fluids  absorbed  by  the  roots  rise  through  the  stem  through  the 
vessels  and  cell  walls  of  the  wood  and  not  through  the  bark. 
The  same  fact  is  demonstrated  by  the  effect  of  girdling  a  tree, 
which  operation  does  not  prevent  the  rise  of  the  sap  nor  cause 
wilting  of  the  leaves. 

78.  Other  functions  of  the  wood  and  bark  will  be  noted 
subsequently. 

Structure  and  Function  of  the  Leaves 

79.  In  order  to  fully  understand  the  function  of  the  leaf  and 
the  important  processes  that  take  place  within  its  tissues  it  is 
necessary  to  study  the  finer  details  of  its  structure  by  means 
of  the  microscope.     Thus  under  moderate  magnification  a  leaf 
seen  in  cross  section  presents  the  following  essential  elements  of 
structure : 

80.  Both  layers  of  epidermis  consist  of  a  single  layer  of  brick- 


STRUCTURE   OF   THE   LEAF 


39 


shaped  colorless  cells,  whose  outer  walls  are  thickened  and 
cutinized,  whereby  they  become  tough  and  impervious.  This 
modification  of  the  cell  wall  is  usually  more  marked  in  the  case 
of  the  upper  epidermis.  The  epidermis — usually  the  lower, 
sometimes  the  upper,  frequently  both — is  pierced  by  numerous 


FIG.  14. — Cross  section  of  a  typical  leaf.     Five  stomata  are  shown  in  the  lower 

epidermis. 

pores,  the  stomata,  which  open  into  a  system  of  intercellular 
spaces  filled  with  air.  The  outside  atmosphere  is  thus  given 
free  access  to  all  parts  of  the  mesophyll  through  the  stomata 
and  this  system  of  intercellular  air  spaces. 

8 1 .  The  more  compact  upper  layer  of  the  mesophyll  consists 
of  cells  elongated  perpendicularly 

to  the  epidermis  and  arranged  in 
ranks,  whence  they  have  received 
the  name  "palisade  cells. "  The 
lower,  spongy  layer  of  the  meso- 
phyll consists  of  cells  less  regular 
in  form  and  arrangement  and 
more  completely  surrounded  by 
air  spaces,  but  otherwise  like  the 
palisade  cells. 

82.  The  important  characteristic 
of  the  cells  of  the  mesophyll  is  the 
presence  of  numerous  minute  green 

granules  embedded  in  the  protoplasm.  The  granules  are  special- 
ized parts  of  the  protoplasm  and  are  called  chloroplasts.  The 
substance  which  gives  them  color  is  called  chlorophyll.  It  is, 


FIG.  15. — Surface  view  of  the 
epidermis  of  a  leaf  showing  several 
stomata.  The  guard  cells  are 
dotted. 


40  PLANTS 

this  chlorophyll  which  gives  green  plants  their  characteristic 
color.  It  may  be  extracted  from  green  tissues  by  alcohol,  in 
which  it  is  soluble,  the  alcohol  then  becoming  green  and  the 
chloroplasts  colorless. 

83.  The  function  of  the  various  parts  of  the  leaf  may  be 
determined  by  suitably  conducted,  simple  experiments. 

84.  The  usual  position  of  the  stomata,  on  the  underside  of  the 
leaf,  indicates  that  the  stomata  are  not  organs  for  the  absorption 


FIG.  1 6. — Stereogram  of  leaf  structure.     Part  of  a  veinlet  is  shown  on  the  right. 
Intercellular  spaces  are  shaded.     (From  Stevens.) 

of  water.  Besides,  most  leaves,  due  to  the  presence  of  a  waxy 
secretion  on  the  surface  of  the  epidermis,  do  not  wet  and,  con- 
sequently, water  would  not  pass  through  the  minute  openings. 
The  function  of  the  stomata  must  be  to  permit  an  interchange 
of  vapors  and  gases  between  the  intercellular  air  spaces  and  the 
atmosphere;  for  by  experiment  it  can  be  shown  that  water  vapor 


TRANSPIRATION 


is  given  off  from  leaves,  but  only  from  the  surface  provided 
with  stomata,  consequently  the  stomata  must  be  regarded  as 
the  openings  through  which  the  vapor  escapes.  The  term 
transpiration  is  applied  to  this  process  by  which  water  in  the 
form  of  vapor  escapes  from  the  leaves. 

85.  If  the  atmosphere  surrounding  a  green  plant  growing  in  a 
closed  chamber  and  exposed  to  the  sunlight  be  tested  from  time 
to  time  for  carbon  dioxide  and 
oxygen  it  will  be  found  that  the 
percentage  of  the  former  gas 
decreases,  while  that  of  the 
latter  increases.  Other  tests 
will  further  show  that  the  car- 
bon dioxide  is  absorbed  and 
assimilated  by  the  leaf,  in  which 
process  an  excess  of  oxygen  over 
that  required  by  the  plant  is  set 
free  in  the  leaf,  and,  if  the  leaf 
is  immersed  in  water,  the  oxy- 
gen may  be  seen  to  collect  on  FlG>  I7_^  Diagram  of  stomain 

the  surface  of  the  leaf  in  bub-     open    and   closed    condition    (heavy 

lines  represent  stoma  open).      B,  C, 
Dies.       ihese  gases  are  not  ab-     and  D,  successive  stages  in  the  de- 


sorbed  or  eliminated    through 

epidermal  surfaces  having   no 

stomata,  consequently  we  must  conclude  that  the  stomata 

give  passage  to  carbon  dioxide  and  oxygen,  as  well  as  to  water 

vapor. 

86.  The  rate  of  transpiration  of  water  vapor  is  controlled 
by  an  automatic  opening  and  closing  of  the  stomata.  Excessive 
transpiration  results  in  the  wilting  of  the  leaf,  which  means  that 
the  cells  having  lost  some  water  are  less  turgid.  The  cells  which 
guard  the  stoma  on  either  side  are  so  constructed  that  with 
increased  turgidity  they  open  the  stoma,  while  with  loss  of 
turgidity  the  stoma  is  closed.  Of  course  the  rate  of  absorption 


42  PLANTS 

of  carbon  dioxide  and  the  accompanying  elimination  of  oxygen 
is  also  dependent  upon  the  opening  and  closing  of  the  stomata. 


Photosynthesis 

87.  The  leaves  of  a  green  plant  growing  under  normal  con- 
ditions always  contain  starch  when  the  plant  has  been  exposed 
to  sunlight  for  a  time.     The  starch  disappears  at  night  or  when 
the  plant  is  placed  in  the  dark.     It  also  disappears  if  the  plant 
is  kept  in  an  atmosphere  which  contains  no  carbon  dioxide. 
Etiolated  leaves  contain  no  starch  under  any  circumstances. 
It  appears  from  these  facts  that  starch  is  formed  in  the  leaf 

only  in  the  presence  of  chlorophyll,  carbon 
dioxide  and  sunlight.  The  chemical  formula 
for  starch  is  CeHioOs,  a  carbohydrate  deriva- 
tive formed  by  the  combination  of  CO2  and 
H20,  thus,  6CO2+5H20  =  C6Hio05+602,  the 
surplus  oxygen  being  given  off  by  the  plant. 
It  will  be  noted  that  the  number  of  molecules 
FIG.  1 8.— Starch  of  oxygen  given  off  equals  the  number  of 

ceanntSr'cOVlinfsCOof     molecules  of  c°2  absorbed,  which  means  that 
growth.  the  volumes  of  the  absorbed  and  eliminated 

gases  are  equal. 

88.  The  power  of  forming  starch  from  inorganic  matter  is  a 
property  peculiar  to  green  plants,  because  of  their  chlorophyll, 
and  gives  them  the  distinction  of  being  the  source  whence  all 
organisms  derive  their  food,  since  starch  is  the  proximate  or- 
ganic form  of  almost  all  food  substances.     The  starch  is  formed 
within,  or  in  contact  with,  the  chloroplasts  and  appears  first  as 
minute  granules  which  grow  by  the  addition  of  layers  to  the 
outside  in  such  a  way  that  the  surface  of  the  fully  formed  grain 
is  marked  by  peculiar  concentric  lines. 

89.  We  now  see  whence  the  plant  derives  its  large  amount 
of  carbon.     From  the  formula  for  starch  it  follows  that  4/9  of 


PHOTOSYNTHESIS  43 

its  weight  is  carbon,  which  is  approximately  the  proportion  of 
carbon  in  the  total  plant  tissue.  The  CO2  of  the  atmosphere 
finds  its  way,  with  the  other  constituents  of  the  air,  through 
the  stomata  of  the  epidermis,  into  the  intercellular  spaces  of 
the  leaf.  From  here  it  passes  through  the  cell  walls  of  the 
mesophyll  by  osmose  and  is  then  by  photosynthesis  converted 
into  starch,  the  free  oxygen  passing  out  of  the  cell,  also  by 
osmose,  to  the  air  of  the  intercellular  spaces  and  thus  out  of 
the  leaf.  This  process  must  not  be  regarded  as  assimilation, 
since  the  substances  absorbed  have  not  been  converted  into 
living  protoplasm  nor  built  up  into  the  structural  elements  of 
the  plant.  The  starch  is  simply  food  material  which  has  been 
manufactured  by  the  plant  from  a  substance  which  is  not  food. 
For  CO2  cannot  be  directly  assimilated  by  protoplasm. 

90.  Starch   is   practically   insoluble   in   water   at   ordinary 
temperature,  yet  it  quickly  disappears  in  an  active  cell  when 
photosynthesis  is  not  going  on.     There  is  an  active  principle 
called  a  ferment  present  in  the  protoplasm,  which  corrodes  the 
starch  grain,  wearing  away  the  surface  until  finally  it  goes  to 
pieces  and  disappears.     In  place  of  the  starch  a  form  of  sugar 
is  found,  dissolved  in  the  cell  sap.     This  is  formed  directly  from 
the  starch  by  the  addition  to  the  molecule  of  a  molecule  of  water, 
thus  starch  (C6HioO5)+H20  =   sugar  (C6Hi2O6),  a  substance 
readily  soluble  in  water.     This  soluble  food  substance  may  be 
directly  assimilated  by  the  protoplasm  of  the  cell  in  which  it  was 
formed,  or  it  may  be  transferred  to  other  cells  by  osmose. 

Respiration 

91.  The  fact  that  oxygen  is  liberated  from  the  plant  during 
photosynthesis   must   not  be  interpreted   to   mean   that  the 
plant  does  not  need  oxygen.     As  has  been  noted  elsewhere, 
oxygen  is  necessary  to  the  germination  of  seeds  and  it  is  as 
necessary  to  the  growing  plant.     During  photosynthesis  more 


44  PLANTS 

oxygen  is  liberated  than  is  needed  by  the  plant  and  the  excess 
escapes.  But  during  the  night,  or  when  for  any  reason  no 
oxygen  is  set  free  in  the  tissues  by  the  synthesis  of  starch,  the 
plant  absorbs  oxygen  directly  from  the  atmosphere.  This  is 
at  all  times  true  of  plants  which  contain  no  chlorophyll.  The 
process  of  absorbing  oxygen  of  whatever  source,  by  vegetable 
tissues,  is  called  respiration  and  is  identical  with  respiration 
in  animals. 

Translocation  of  Food  Substances 

92.  The  midribs  and  veins  of  the  leaf  are  continuations  of 
the  vascular  bundles  of  the  stem.     Besides  giving  support  to 
the  softer  tissues  they  also  bring  the  leaf  into  communication 
with  the  rest  of  the  plant  through  the  vascular  system,  per- 
mitting the  passage  of  liquids  and  gases  between  the  leaf  and 
the  stem. 

93.  Since  starch  and,  consequently,  sugar,  are  formed  only 
in  cells  containing  chlorophyll,  all  other  cells  must  be  dependent 
for  their  food  upon  those  which  contain  chlorophyll.     Conse- 
quently, in  the  larger  number  of  plants,  the  leaves  must  elabo- 
rate all  the  food  for  the  stem  and  root.     Starch  is  frequently 
found  in  parts  containing  no  chlorophyll.     In  such  cases  it 
has  been  formed  from  sugar  by  the  action  of  colorless  corpuscles, 
called  amyloplasts,  which  differ  from  chloroplasts  only  in  the 
absence  of  chlorophyll.     The  course  of  the  sugar  through  the 
stem  is  chiefly  along  sieve  vessels  and  the  surrounding  paren- 
chyma.    In  passing  from  cell  to  cell  it  is  frequently  converted 
into  starch  and  then  reconverted  into  sugar  preparatory  to  the 
next  osmotic  transfer. 

Other  Food  Substances 

94.  Besides  the  carbohydrates,  starch  and  sugar,  there  are 
several  other  kinds  of  food  substances  elaborated  in  the  leaf. 


OIL,   ALEURONE 


45 


=-    A 


B 


FIG.  19. — Section  of  a  grain 
of  wheat.  A,  Pericarps  and 
seed  coats;  B,  layer  of  cells  in 
the  endosperm  containing 
aleurone  grains;  C,  cells  of  the 
endosperm  containing  starch 
grains. 


Prominent  among  these  are  the  various  vegetable  oils,  which  are 
also  compounds  of  carbon,  hydrogen  and  oxygen.  Globules 
of  oil  may  be  found  in  the  tissues  of  the  leaf  and  in  other  parts 
of  the  plant,  but  it  is  especially  in  the  seeds  of  certain  plants 
that  large  quantities  of  oil  are  stored 
up,  to  serve  the  same  purposes  that 
is  served  by  starch  in  other  cases. 

95.  Aleurone  is  a  substance  which 
contains,  besides   carbon,    hydrogen 
and  oxygen,  also  a  small  per  cent, 
of  nitrogen.     It  is  therefore  called  a 
nitrogenous   substance   and  is   very 
much  like  albumen.     It  is  soluble  in 
water  and   consequently   disappears 
when  immersed  in  a  watery  solution, 
but  by  mounting  tissues  of  dry  seeds 
containing  it  in  a  medium  like  glyc- 
erine, the  aleurone  may  be  seen  under  the  microscope  in  the 
form  of  small  granules. 

Differentiation  of  Tissues 

96.  All  reserve  food  materials  are  ultimately  converted  into 
protoplasm    and    from    protoplasm    the    various    structural 
elements  of  the  tissues  are  formed.     Thus  the  undifferentiated 
cell  walls  of  parenchyma  consist  of  cellulose,  C6Hi0O5,  a  sub- 
stance having  the  composition  of  starch.     The  cellulose  is 
formed  from  layers  of  protoplasm  by  a  process  of  chemical 
transformation. 

97.  By  further  alteration  in  the  chemical  nature  of  the  cellu- 
lose walls  by  which  the  proportion  of  carbon  is  increased,  the 
walls  assume  special  characteristics ;  the  surface  wall  of  epider- 
mal cells  becomes  cutinized  (cutin),  the  walls  of  cork  cells  be- 
come suberized  (suberin),  and  the  walls  of  wood  and  bast 
fibres  become  lignined  (lignin). 


46 


PLANTS 


98.  All  parts  of  the  plant  are  covered,  at  least  during  the 
early  stages  of  their  development,  by  a  superficial  layer  of 
cells  forming  the  epidermis.     On  parts  exposed  to  the  air  the 
outer  walls  of  the  epidermal  cells  are  cutinized,  which  renders 
them  impervious   and   tough.     These  properties   render   the 
epidermis  well  fitted  to  prevent  desiccation  of  the  underlying 
tissues  and  to  protect  them  from  mechanical  injury. 

99.  On  the  smaller  rootlets,  which  are  always  surrounded 
by  the  moist  soil  and  hence  not  subject  to  either  desiccation 


*  ^  *"**'" 


FIG.  20. — The  epidermis  of  various  plants  showing  different  degrees  of  cu- 
tinization  (in  black).  A,  Leaf  of  Avicennia,  a  Xerophyte;  B,  the  epidermis  of 
an  apple  (fruit);  C,  petal  of  Japan  quince;  D  and  E,  upper  and  lower  epidermis  of 
leaf  of  Hibiscus  Moscheutos;  F,  epidermis  of  leaf  of  prickly  lettuce,  Lactuca 
scariola,  in  the  sun;  G,  same,  in  the  shade.  (From  Stevens.) 

or  mechanical  injury,  the  epidermis  is  not  cutinized,  conse- 
quently it  offers  no  obstacle  to  the  transfusion  of  water  and  in 
fact  is  here  specially  modified  for  the  function  of  absorption 
through  the  medium  of  the  root  hairs,  which  are  only  expansions 
of  some  of  the  epidermal  cells. 

100.  Structures  called  hairs  are  also  developed  on  aerial 
parts  of  the  plant.  These  assume  as  endless  variety  of  forms 
and  serve  various  functions.  Some  are  glandular,  others  are 
organs  for  water  absorption,  and  still  others  serve  a  variety  of 


EPIDERMIS 


47 


special  functions.  Most  of  those  found  on  the  leaf  and  stem, 
however,  must  be  classed  with  protective  structures,  protecting 
the  parts  they  cover  from  too  intense  sunlight,  too  rapid  trans- 
piration, attacks  of  animals,  wetting  and  frosts,  etc. 

101.  The  epidermis  is  elastic  and  stretches  to  a  remarkable 
degree  as  the  parts  covered  by  it  expand  with  growth.     But 
on  the  roots  and  stems  of  perennials 

the  limit  of  elasticity  is  reached  after 

a   few   years   and   then   the  epidermis 

gives   way,   breaking  in  various  ways 

and  exposing  the  tissues  beneath.     Its 

place  as  a  protective  structure  is  taken 

by  the  underlying  layers  of  the  bark 

which  have  then  become  modified  into 

cork.     This  serves  more  efficiently  the 

function   of   protection   than   did    the 

epidermis,   though   at   the   expense   of 

depriving  the  tissues  beneath  of  the 

sunlight  which  had  before  been  trans- 

mitted  by  the  transparent  epidermis.       FlG.  „._,,  Hooked  hair 

The  corky  layers  are  thick  and  opaque,    from  the  stem  of  Phaseolus 

i  , .  i        multiflorus:  2,  climbing  hair 

though  at  the  same  time  extremely  on  stem  of  Humulus  Lupu- 
impervious,  extremely  poor  conductors  lus>  3,  rod-like  wax  coating 

r  '    r .  on  stem  of  Saccharum  offic- 

Of    heat,    not    readily    yielding    to    the    inarum;     4,   climbing    hair 

claw  or  tooth  of  beast  or  the  beak  of 
bird,  almost  valueless  as  food  for 
animals  and  offering  an  excellent  pro- 
tection against  the  attacks  of  fungous  parasites.  The  outer 
layers  of  the  cork  are  dead  tissue,  which  usually  splits  into 
ridges  as  the  stem,  expands,  and  later  the  outer  layers  even 
scale  off  and  drop  away,  while  new  layers  are  constantly 
forming  beneath  from  the  cork  cambium. 

102.  The  most  delicate  tissues  of  the  plant  are  those  in  which 
growth  is  taking  place  by  the  multiplication  of  cells.     The  three 


of  Losa  hispida;  5,  stinging 
hair  of  Urtica  ureus.  (From 
Stevens,  after  deBary  and 
Haberlandt.) 


PLANTS 


chief  regions  in  which  this  occurs  are  the  centre  of  the  bud, 
the  cambium  layer  of  the  stem  and  roots  and  the  root  tip.     The 


FIG.  22. — Diagram  to  show  the  development  of  the  tissues  (differentiation' 
near  the  tip  of  a  growing  stem.  The  four  figures  on  the  right  represent  cross 
sections  at  different  distances  from  the  tip  of  the  stem.  The  figure  shoulc 
represent  the  tip  of  the  stem  covered  by  the  young  leaves.  (From  Stevens. ] 

growing  tissue  of  the  bud  lies  at  the  centre  of  a  group  of  oldei 
structures  and  hence  is  not  exposed  except  that  the  bud  as  a 


ROOT   CAP 


49 


whole  may  suffer  injury;  and  the  cambium  lies  beneath  the  bark 
which  gives  it  ample  protection.  But  the  tip  of  the  root,  as 
it  grows,  must  push  its  way  through  the  harsh  soil  and  is  there- 
fore provided  with  a  special  protective  structure,  the  root  cap. 
This  is  a  conical  mass  of  cells 
fitting  over  the  tip  of  the  root. 
As  the  rootlet  pushes  forward 
through  the  soil  some  of  the  cells 
of  the  root  cap  are  rubbed  off 
or  destroyed,  while  others  from 
beneath  take  their  places,  new 
ones  being  continually  formed 
for  this  purpose  at  the  point  of 
growth  in  the  base  of  the  cap. 

Modified  Roots 

103.  In  many  plants,    espe- 
cially among  biennials  and  per- 
ennials, the  roots  show  peculiar- 
ities of  form  and  structure  which 
cannot  be  accounted  for  with 
reference  to  the  usual  functions 
of  roots,  viz.,  those  of  absorption 
and  anchorage.     These  modifi- 
cations are  often  in  the  nature 
of  enlargements,  as  in  the  case 
of  the  turnip  and  sweet  potato. 

year  such  roots  give  rise  to  new  shoots  from  undeveloped  or 
adventitious  buds.  The  root  shrivels  as  the  shoot  grows 
because  of  the  gradual  absorption  of  the  contained  food  store, 
the  enlargement  being  due  to  the  accumulation  of  starch  or 
other  elaborated  food  substances. 

104.  A  less  common  type  of  root  is  the  prop  root,  which 

4 


FIG.  23. — Longitudinal  section  of 
the  tip  of  a  rootlet  with  the  root  cap. 
The  lower  third  of  the  figure  is  the 
cap.  The  region  of  growth  (multi- 
plication of  cells)  is  indicated  by  the 
small  size  of  the  cells.  The  black 
dots  are  the  nuclei. 


In  the  spring  of  the  second 


50  PLANTS 

springs  from  the  stem  above  ground,  or  even,  in  some  cases, 
from  the  branches,  and  grows  down  to  and  into  the  soil.  Such 
roots  are  found  in  special  cases  in  which  the  plant  would  other- 
wise be  top-heavy  for  its  basal  root  system.  The  prop  roots 
are  primarily  for  anchorage  though  they  may  also  serve  for 
absorption. 

105.  Another  form  of  modified  root  is  found  in  certain  climb- 
ing plants  which  have  roots  springing  from  the  aerial  parts  of 
the  plants.     These  aerial  roots  serve  as  hold-fasts,  penetrating 
the  superficial  layers  of  the  bark  of  the  tree,  or  crevices  of  the 
rock,  to  which  the  plant  clings  for  support. 

1 06.  The  function  of  the  aerial  roots  of  epiphytes,  so  common 
in  humid  climates,  is  not  only  to  attach  the  plant  to  its  host 
but  also  to  absorb  moisture.     In  some  cases  the  moisture  is 
absorbed  directly  from  the  atmosphere;  in  others,  it  is  drawn 
from  the  sponge  of  decaying  leaves  and  other  vegetable  sub- 
stance which  collects  among  the  tangled  mass  of  roots.     In  the 
latter  case  the  absorbed  water  is  likely  to  contain  more  or  less 
nourishing  matter,  extracted  from  the  humus. 

Modified  Stems  and  Branches 

107.  Stems  are  frequently  so  much  reduced  in  length  that 
the  leaves  seem  to  spring  directly  from  the  roots.     In  such 
"stemless  plants,"  however,  the  conical  or  disc-shaped  surface 
from  which  the  leaves  arise  must  be  regarded  as  the  stem,  at  the 
apex  or  centre  of  which  the  terminal  bud  will  always  be  found. 
Many  biennials  remain  "stemless"  during  the  first  season,  but 
during  the  second  period  of  growth  produce  a  normal  stem 
and  branch  system  by  development  from  the  terminal  bud. 

1 08.  Another  type  of  stem  is  that  characteristic  of  the  climb- 
ing and  trailing  plants.     In  these  the  stem  is  too  slender  to 
maintain  itself  in  an  erect  position.     The  climbers  depend  on 
other  objects  for  support,   the  stem  serving  merely  as  the 
conducting    system    connecting    roots    and    leaves.     In     the 


MODIFIED    STEMS  51 

case  of  trailers  the  plant  is  enabled  to  secure  a  large  light 
exposure  by  spreading  over  a  large  surface  of  ground.  The 
stem,  in  this  case  also,  serving  only  the  function  of  conduction. 
Climbers  from  their  habit  are  adapted  to  forested  regions, 
while  trailers  flourish  in  open  ground. 

109.  In  many  cases  trailing  stems  take  root  at  the  nodes. 
Such  stems  are  called  runners,  or  stolons.  The  object  of  such 
a  habit  may  be  simply  supplementary  to  the  function  of  the 
basal  root  system  or  else,  if  the  stem  also  produces  a  system 
of  branches  at  the  nodes,  it  may  result  in  the  production  of 
new  plants — an  asexual  method  of  reproduction.  In  this  case, 
after  the  young  plant  has  become  firmly  established  the  stolon 
connecting  it  with  the  parent  may  die,  leaving  the  young  plant 
independent.  If  the  connecting  stolons  persist  it  is  possible 
that  the  associated  individuals  may  be  of  mutual  physiological 
assistance  at  critical  times  in  the  way  of  furnishing  each  other 
nourishment,  etc.  It  is  certainly  true  that  such  plants  growing 
on  a  shifting  soil  are  of  great  mutual  assistance  in  holding  each 
other  in  place  and  thereby  also  holding  the  soil.  Stolons  may 
be  either  above  or  under  the  ground. 

no.  Underground  stolons  are  sometimes  greatly  enlarged 
at  the  end,  forming  tubers.  This  is  due  to  the  accumulation  of 
food  substances  for  the  purpose  of  storage,  which  is  one  of  the 
normal  functions  of  the  stems  of  perennial  plants.  In  the  larger 
perennials  the  normal  stem  is  large  enough  to  provide  sufficient 
storage  and  consequently  no  special  enlargement  is  necessary. 
In  the  case  of  the  smaller  herbaceous  perennials,  however,  there 
is  insufficient  storage  room  provided  by  the  comparatively 
small  normal  stem  and  therefore  the  storage  stems  are  enlarged. 
Moreover,  since  the  normal  herbaceous  stems  usually  do  not 
survive  the  winter,  the  stems  which  are  modified  for  storage  are 
developed  underground,  where  they  are  protected,  and  whence 
they  put  forth  shoots  in  the  following  spring  from  terminal 
and  axillary  buds. 


52  PLANTS 

in.  Other  types  of  underground  storage  stems  are  common. 
The  root-stock  differs  from  the  tuber  in  that  it  has  no  slender 
connecting  stem  but  is  thickened  throughout  its  length.  It 


FIG.  24. — Young  shoots  of  the  common  cactus,  Opuntia,  showing  the  small, 
conical  leaves.  These  soon  disappear.  Note  that  the  spines  develop  in  the 
axils  of  the  leaves.  X2/3. 

usually  persists  from  year  to  year;  a  new  segment  consisting 
of  one  or  more  nodes,  being  added  each  season.     When  the 


STORAGE    STEMS  53 

root-stock  is  much  shortened  and  vertical  in  position — in  other 
words,  merely  an  extremely  short  stem — it  becomes  a  corm. 

112.  Many  plants  inhabiting  semi-arid  regions  are  adapted 
to  the  recurring  long  periods  of  drought  following  the  brief 
periods  of  rainfall  by  the  habit  which  they  have  assumed  of 
storing  up  water.     The  stems  form  the  reservoirs  and  are  con- 
sequently of  much  greater  bulk  than  the  other  functions  of  the 
stem  would  demand. 

113.  A  less  common  type  of  modified  stem  is  one  in  which 
the  branch  takes  up  the  functions  of  the  leaf.     In  this  case  the 
branch  may  become  flattented  and  like  the  leaf  in  other  respects. 
Thorns  in  certain  cases  are  also  modified  branches. 

Modified  Leaves 

114.  Besides  the  endless  diversity  of  form  assumed  by  foliage 
leaves,  there  are  also  a  number  of  leaf  types  in  which  the  function 
of  photosynthesis  has  been  entirely  lost.     Such,  for  example, 
are  the  bud  scale-leaves,  which  serve  as  protective  organs,  and 
the  scale-leaves  of  underground  stems,  which  are  functionless 
rudiments.     Certain  kinds  of  thorns  and  tendrils,  are  modified 
leaves  or  parts  of  leaves.     Even  the  function  of  food  storage 
is  sometimes  assumed  by  leaves.     The  blade  of  the  leaf  in  one 
group  of  plants  is  entirely  wanting  and  its  function  is  performed 
by  the  petiole,  which  is  flattened  laterally  and  has  the  appear- 
ance of  a  leaf  blade  turned  into  the  vertical  plane. 

Homology  of  the  Flower 

115.  While  the  modification  of  the  type  forms  mentioned 
in  the  preceding  paragraphs  are  all  quiet  common,  still  they  are 
in  every  case  limited  to  a  small  minority  of  plant  species.     There 
is,  however,  a  most  important  and  interesting  kind  of  modi- 
fication which  is  practically  universal  among  seed-bearing  plants. 


54  PLANTS 

This  is  the  modification  of  a  branch,  involving  both  stem  and 
leaves,  which  results  in  the  structure  we  call  the  flower. 

1 1 6.  A  leaf  bud  and  a  flower  bud  are  in  all  essential  points 
alike.     There  is  a  very  short  central  axis  around  which  are 
arranged  the  rudimentary  leaves  in  regular  whorled  or  spiral 
order.     In  the  development  of  the  leaf  bud  the  axis  elongates, 
separating  the  leaves,  while  the  latter  expand  and  assume  the 
form  and  color  of  the  typical  leaf.     In  the  case  of  the  flower 
bud,  however,  the  axis  does  not  elongate  regularly  throughout 
its  length.     It  may  remain  very  short,  in  which  case  the  flower 
remains  sessile.     If  the  axis  elongates  at  all  the  elongation 
affects  only  a  limited  part,  by  which  a  stem  (pedicel  or  peduncle) 
is  formed.     At  the  top  of  this  stem  the  flower  leaves  still  remain 
in  closely  set  whorls  or  circles. 

Inflorescence 

117.  The  homology  of  flowers  is  also  shown  by  their  position 
on  the  stem  and  their  groupings.     When  flowers  occur  singly 
they  are  either  terminal  or  axillary  and  hence  arise  from  ter- 
minal or  axillary  buds,  or  else  they  spring  from  accessory  buds. 
In  either  case  their  origin  is  the  same  as  that  of  branches. 
Whenever  a  flower  terminates  an  axis  the  growth  of  that  axis 
ceases  with  the  growth  of  the  flower,  consequently  further 
growth  of  the  plant  must  proceed  from  another  bud. 

118.  Flowers  which  occur  in  groups  may  be  divided  into  two 
classes,  depending  upon  whether  the  first  flower  to  appear  is 
terminal  or  lateral.     In  the  former  the  grouping  of  the  flowers 
is  called  a  determinate  or  cymose  inflorescence.     In  this  case 
the  first  terminal  flower  is  followed  by  two  opposite,  lateral 
ones  which  grow  beyond  the  first,  leaving  it  apparently  in  the 
angle  of  two  equal  lateral  branches.     This  is  a  simple  cyme. 
If  the  two  lateral  flower  stalks  also  each  put  out,  in  a  similar 
way,  a  pair  of  lateral  flowers,  the  cyme  becomes  compound. 


INFLORESCENCES 


55 


119.  When  the  first  flower  of  an  inflorescence  is  lateral  the 
terminal  bud  continues  to  grow  for  some  time  and  new  flowers 


a* 


\/  Nl/ 

\ 


FIG.  25. — Cymose  inflorescences.     F,  A  terminal  flower;  G,  a  simple  cyme;  H,  a 

compound  cyme. 


' 

?     c 

0 

^ 

3                 / 

c 

p 

p 

^) 

'             c 

/"    c 

p 
p 

b 
5 

^c 

p 

0 

5 

A 

B 

'c 

FIG.  26. — Types  of  racemose  inflorescence.  A,  A  raceme;  B,  a  spike;  C,  a 
catkin;  D,  a  corymb;  £,  an  umbel.  The  flowers  are  represented  by  circles; 
the  age  of  the  flower  is  indicated  by  the  size. 

continue  to  develop  above  the  first  one  along  the  main  axis. 
Such  an  inflorescence  is  indeterminate  and  is  called  a  raceme. 


56  PLANTS 

If  the  flowers  of  the  raceme  are  sessile  the  inflorescence  is  a  spike; 
if  they  are  stalked  it  is  a  true  raceme.  A  scaly,  pendulous, 
deciduous  spike  is  a  catkin.  If  the  older  flowers  of  a  raceme 
rise  to  the  level  of  the  terminal  one  because  of  their  longer  stalks, 
and  thus  form  a  flat  topped  cluster,  we  have  a  corymb.  When 
the  rachis  is  much  shortened  and  the  flowers  equally  stalked 
the  inflorescence  is  an  umbel  and  a  similar  condition  of  the 
rachis  with  sessile  flowers  is  a  capitulum. 

120.  That  an  inflorescence  is  made  up  of  a  system  of  branches 
is  further  shown  by  the  fact  that  each  flower  springs  from  the 
axil  of  a  bract,  or  rudimentary  leaf.     These  are  often  green, 
but   sometimes   scale-like   or   chaffy.     There  is   frequently   a 
series  of  such  bracts  at  the  base  of  an  inflorescence  forming  an 
involucre.     This  is  especially  true  of  capitulate  inflorescences. 

Structure  of  the  Flower 

121.  A  complete  flower  has  four  sets  of  floral  leaves,  all  more 
or  less  completely  transformed  for  special  functions  and  bearing 
little  resemblance  to  the  foliage  leaf.     The  extreme  diversity 
of  species  with  respect  to  the  characteristics  of  the  flower  makes 
it  impossible  to  give  any  general  description  of  it  which  will 
apply  to  all  cases.     However,  an  ideal  flower  with  which  all 
others  may  conveniently  be  compared  may  be  described  as 
follows: 

122.  That  part  of  the  flower  stem  which  bears  the  leaves  is 
so  much  shortened  that  it  forms  practically  a  flat  surface,  the 
receptacle,  upon  which  are  borne  the  concentric  circles  of  floral 
leaves.     The  outermost  or  lowest,  of  these  circles  is  called  the 
calyx.     It  is  formed  of  from  three  to  five  leaves,  which  are  green 
and  enclose  the  other  parts  in  the  bud.     The  next  circle  within 
this  is  composed  of  a  similar  number  of  parts,  characterized  by 
some  color  other  than  green.     This  circle  is  called  the  corolla, 
and  corolla  and  calyx  together  are  sometimes  called  the  peri- 


THE   FLOWER 


57 


anth.  Neither  calyx  nor  corolla  are  essential  parts  of  the  flower. 
One  or  both  may  be  wanting  without  thereby  impairing  the 
function  of  the  flower. 

1 23 .  The  third  circle  constitutes  the  andrcecium  and  is  made 
up  of  parts  called  stamens,  which  ordinarily  have  little  resem- 
blance to  leaves.  There  is  usually  a  slender  stalk  (the  filament) 


FIG.  27. — Diagrams  of  floral  structures.  A  shows  the  relations  of  the  floral 
parts  in  a  hypogynous  flower;  B,  the  same  in  a  perigynous  flower;  C,  the  same  in 
an  epigynous  flower;  D,  a  stamen;  E,  a  simple  pistil  in  longitudinal  section;  F, 
the  same  in  cross  section;  G,  transitional  forms  between  true  petals  (left)  and 
true  stamens  (right);  H,  slight  union  of  two  carpels  to  form  a  compound  pistil; 
7  and  /,  union  of  carpels  more  complete;  K  and  L,  cross  sections  of  compound 
pistils,  of  three  carpels.  In  B:  a,  stamen;  b,  petal;  c,  sepal;  d,  pistil;  e,  receptacle; 
/,  pedicel.  In  D:  a,  anther  cell;  b,  connective;  c,  filament.  In  E:  a,  stigma; 
b,  style;  c,  ovules;  d,  ovary. 

at  the  summit  of  which  is  attached  a  double  sack-like  organ 
(the  anther)  containing  a  powdery  or  granular  substance  (the 
pollen) . 

124.  In  certain  flowers  the  stamens  bear  a  close  resemblance 
to  a  leaf.  In  such  cases  the  filament  is  leaf-like  in  form  and  the 
cells  of  the  anther  are  borne  on  its  edge.  The  number  of  sta- 
mens is  frequently  the  same  as,  or  a  multiple  of,  the  number 
of  parts  of  the  calyx  or  of  the  corolla,  but  it  may  vary  from  one 
to  many. 


PLANTS 


125.  The  gyncecium  is  the  organ  or  set  of  organs  formed  by 
the  fourth  or  inner  circle  of  floral  leaves.  The  individual 
leaves  (carpels)  composing  it  may  be  more  or  less  united  to 
form  a  single  structure  or,  not  infrequently,  the  number  of 
carpels  may  be  reduced  to  one,  which  then  occupies  the  centre 
of  the  flower.  In  case  there  is  only  one  carpel,  or  if  the  carpels 
are  separate,  each  one  constitutes  a  simple 
pistil,  which  must  be  regarded  as  having 
been  formed  by  the  rolling  of  the  blade  of 
the  carpellate  leaf,  so  that  its  opposite 
edges  meet  and  unite  and  thus  enclose 
a  flask-shaped  cavity.  The  pistil  thus 
formed  may  be  described  as  consisting 
of  the  ovary — the  cavity  of  the  flask  with 
its  enclosing  walls,  the  style — the  neck  of 
the  flask — and  the.  stigma,  a  slight  glandu- 
lar enlargement  at  the  top  of  the  style. 
(See  Fig.  27.) 

126.  Within  the  cavity  of  the  ovary 
and  attached  to  its  walls  are  one  or  more 
minute  bodies,  the  ovules,  which  are 
destined  to  develop  into  the  seed.  The 
specialized  part  of  the  ovary  wall  to  which 
the  ovules  are  attached  is  the  placenta. 
(See  Fig.  27.) 

127.  Pistils  are  frequently  compound,  i.  e.,  made  up  of  more 
than  one  carpel.     In  such  cases  there  may  be  various  degrees 
of  fusion  of  the  component  leaves,  ranging  on  the  one  hand  from 
a  slight  external  union  of  the  ovary  walls  to  such  complete 
fusion  on  the  other,  that  the  only  evidence  of  its  compound 
nature  is  to  be  found  in  the  number  of  placentae.     The  number 
of  pistils  in  one  flower  varies  from  one  to  many. 

128.  Both  andrcecium  and  gyncecium  are  essential  and  with- 
out either  one  the  flower  is  incapable  of  performing  its  function. 


FIG.  28.— One  of  the 
four  leaf-like  carpels  of 
the  Chinese  parasol  tree 
(Sterculia)  with  several 
seeds  attached  to  its 
margins.  The  carpels 
separate  early  and  as- 
sume a  leaf-like  form. 


THE    FLOWER 


59 


There  are  many  plants,  however,  in  which  stamens  and  pistils 
are  not  found  in  the  same  flower.  In  such  cases  there  are 
two  kinds  of  flowers,  one  staminate,  the 
other  pistillate,  both  found  on  the  same 
plant  (monoecious)  or  separate  plants 
of  the  same  species  (dioecious). 

129.  The  number  of  deviations  from 
the  ideal  flower  just  described  are  too 
many  to  be  enumerated,  but  it  will  be 
necessary  to  indicate  the  most  impor- 
tant ones  in  order  that  homologies  may 
be  recognized. 

130.  The   receptacle  may  be  either 
convex,  flat,  or  concave.     In  the  latter 
case  the  edges  of  the  receptacle  may 
rise  so  high  around  the  gyncecium  as 

to  entirely  enclose  it  within  the  con- 

J  m 

cavity.      (See  Fig.  27.) 

131.  The  calyx  is  very  rarely  want- 
ing  since  its  function  is  that  of  protec- 
tion. 

sepals,  but  frequently  there  is  a  more 


FIG.   29. — Diagram  of  a 
pistil  with  one  ovule  in  the 


attached  by  a  stalk,  the 

funiculus,  and  is  provided 

Its  parts   may   all  be  separate    with  two  protective  layers, 


or  less  complete  union  of  the  edges  of  the  seed  coats.    The  em- 

,,  i        -.LI  i      .LI  r  bryo  is  developed  from  the 

the  sepals  with  each  other  so  as  to  form  egg  nucieus  (small  circle) 

a  cup  or  tube  (calyx  gamosepalous)  .  whi(*  lies  in  the  embryo 

/  sac  (large  oval).     The  em- 

132.   The     corolla     is    often    entirely  bryo  sac  is  embedded  in  a 

wanting.     In  other  cases  it  is  present,  ^  *&£&£• 

but   inconspicuous.      Usually,  however,  bryo   is   nourished  by   the 

.  .  11      .  .     •  contents  of  the  embryo  sac 

when  the  corolla  is  present  it  is  very  and  the  nucellus.    Reserve 
because   of   the  size  and 


conspcuous 

color    of    its   parts.      Like    the   Calyx   it     sperm  while  that  found  in 
i  !  -  c    i.   ..  the  nucellus  is  perisperm. 

may  be  made  up  of  distinct  parts,  or 

the  parts  may  be  more  or  less  united  into  a  single  structure 

(corolla  gamopetalous)  . 


60  PLANTS 

133.  The  stamens  may  also  be  distinct  or  united  into  one 
(monodelphous) ,  two  (diadelphous)  or  more  groups,  the  union 
being  due  to  the  cohesion  of  either  filaments  or  anthers. 

134.  A  union  of  floral  organs  occurs,  not  only  between  mem- 
bers of  the  same  series,  but  also  between  adjacent  series.     Thus, 
the  petals  may  be' united  with  the  calyx  cup  in  such  a  way  that 
they  seem  to  spring  from  the  edge  of  the  cup  instead  of  from 
the  receptacle.     The  stamens,  likewise,  may  be  adnate  to  the 
petals  or  fused  with  the  calyx  tube  and  thus,  like  the  petals, 
apparently  inserted  upon  it  (perigynous) .     Still  greater  fusion 
may  occur  and  calyx,  corolla,  and  stamens  all  be  more  or  less 
united  with  the  ovary  and  thus  apparently  inserted  on  its  side 
(perigynous)  or  top  (epigynous)  instead  of  on  the  receptacle. 
In  the  latter  case  the  ovary  is  said  to  be  inferior. 

Function  of  the  Flower 

135.  The  function  of  the  flower  is  to  produce  the  seed,  but 
this  is  accomplished  only  by  the  conjoint  action  of  pollen  and 
ovule.     Under  normal  conditions,  the  ovule  at  a  certain  time 
begins  a  series  of  developmental  changes  by  which  it  finally 
becomes  a  seed.     This  latter  phase  of  its  development  is  begun, 
however,  only  after  pollination  and  fertilization.     Pollination 
is  the  transfer  of  pollen  from  the  anther  to  the  stigma  by  the 
wind,  by  insects,  or  through  some  other  agency.     After  reaching 
the  stigma  the  pollen  grain  develops  a  tubular  outgrowth  which 
penetrates  the  tissues  of  the  stigma  and  style  growing  down 
to  and  into  the  ovule.     A  certain  nucleus  of  the  pollen  tube 
then  fuses  with  a  similar  nucleus  of  the  ovule.     This  fusion  of 
elements  from  the  pollen  grain  and  ovule  is  known  as  fertili- 
zation, because,  as  a  result  of  it,  the  ovule  is  stimulated  to 
further  development  which  finally  results  in  the  seed,  whereas, 
if  the  fusion  does  not  occur,  there  is  no  further  development  of 
the  ovule  and  no  seed  is  produced. 


FUNCTION   OF   THE   FLOWER  6 1 

Pollination 

136.  A  necessary  preliminary  to  fertilization,  however,  is 
pollination,  which  is  brought  about  in  many  different  and  often 
remarkable  ways,  all  of  which  illustrate  most  clearly  the  nice 
adaptations,  or  correlations,  of  plants  with  other  organisms  and, 
in  general,  with  their  environment. 

137.  It  may  first  be  noted  that  most  flowers  are  so  organized 
as  to  effectually  protect  their  pollen  from  wetting.     This  means 
in  many  cases  merely  that  the  flowers  do  not  open  except  in 
fair  weather  and  then  require  only  a  few  minutes  or  hours 
for  the  accomplishment  of  pollination.     Other  flowers  close 
at  night  or  during  threatening  weather,  i.  e.,  the  petals  assume 
a  position  such  as  to  protect  the  stamens  from  rain  or  dew. 
In  other  cases  the  petals,  some  or  all,  are  so  disposed  as  to  give 
shelter  to  the  stamens;  and  frequently  the  flowers  are  pendant, 
so  that  the  stamens  are  sheltered  even  when  the  petals  are  widely 
spread. 

138.  It  is  a  well  recognized  biological  principle  that  cross- 
fertilization,  i.  e.,  fertilization  by  pollen  from  another  plant  of 
the    same    species,    results    in    more  vigorous  offspring  than 
does  self-fertilization — fertilization  resulting  from  the  union  of 
elements  of  the  same  plant.     Accordingly  we  find  that  plants 
are  so  organized  as  to  favor  cross-fertilization. 

139.  One  of  the  most  important  agencies  of  pollination  is 
the  wind.     The  plants  for  which  the  wind  performs  this  service 
all  have  small  and  inconspicuous  flowers,  i.  e.,  the  petals  are 
either  wanting  or,  if  present,  are  small  and  not  brilliantly 
colored.     The  pollen  in  such  plants  is  light  and  powdery  and  is, 
therefore,  easily  carried  by  the  wind,  sometimes  to  long  dis- 
tances.    It  is  produced  in  great  quantities  and,  as  it  is  wafted 
along  on  the  wind  in  clouds,  some  grains  are  likely  to  fall  upon 
other  flowers  of  the  same  species  and  be  held  there  by  the 
adhesive  stigma. 


62 


PLANTS 


140.  To  the  fact  that  anemophilous  flowers  are  inconspicuous 
must  be  added  the  evidently  related  facts  that  such  flowers  are 


FIG.  30. — The  inflorescence  of  Polygala.  The  flowers  are  clustered  in  the  axils 
of  the  whorls  of  leaf-like  bracts.  These  bracts  are  violet  colored  and  render  the 
inflorescence  very  conspicuous. 

also  devoid  of  odor  and  secrete  no  honey.  These  facts  become 
significant  when  we  learn  further  that  all  flowers  which  are 
conspicuous  because  of  the  color  of  the  corolla  or  other  parts, 


POLLINATION  BY   INSECTS  63 

or  are  scented,  or  secrete  honey,  have  the  office  of  pollination 
performed  for  them  by  insects  which  visit  them  for  the  pollen 
or  honey;  the  pollen  as  well  as  the  honey  being  used  by  insects 
as  food.  The  colors  and  scents  of  flowers  are  evidently  related 
to  the  senses  of  sight  and  smell  of  insects  and  serve  to  attract 
the  insects  to  them. 

141.  Insects   on   visiting    the   flowers   necessarily   come   in 
contact  with  the  anthers  and  some  of  the  pollen  clings  to  the 


FIG.  31. — The  dichogamous  flowers  of  Polygala.  The  flower  A  shows  the 
anthers  protruding  from  the  hood.  In  B  the  stigma  has  advanced  and  is  ready 
for  pollination. 

insect's  body;  for  the  character  of  the  pollen  grains  in  such 
plants  differs  from  the  powdery  pollen  of  anemophilous  plants 
in  that  it  is  sticky  by  virtue  of  a  viscid  or  oily  coating,  or 
because  of  the  prickles,  grooves,  ridges  or  other  structural 
peculiarities  of  the  wall  of  the  pollen  grain  which  cause  it  to 
ciing  more  readily  to  the  hairs  of  the  insect's  body.  Now  as 
me  insect  moves  on  to  another  flower  it  will  in  all  probability 


64 


PLANTS 


FIG.  32. — Bumblebees  and  wasps  which  carry  pollen  for  Polygala.     The  hairs  on 
the  thorax  of  all  these  insects  were  covered  with  pollen. 


CROSS   POLLINATION  65 

brush  off  some  of  this  pollen  upon  the  parts  of  the  flower  with 
which  it  comes  in  contact.  Because  of  its  position  and  because 
of  the  character  of  the  stigmatic  surface,  the  stigma  will  be 
most  likely  to  receive  and  retain  some  of  this  pollen,  and  thus 
pollination  will  be  accomplished. 

142.  The  question  now  arises,  is  not  pollination  by  wind  or 
by  insects  as  likely  to  result  in  self-fertilization  as  in  cross- 
fertilization?     The  study  of  further  facts  will  lead  us  to  answer 
this  question  emphatically  in  the  negative.     The  facts  are  these : 
In  the  case  of  anemophilous  plants  the  flowers  are  either  dioe- 
cious, monoecious,  or  dichogamous.     In  the  first  case,  of  course, 
self-fertilization   cannot   occur.     In    the   case   of   monoecious 
plants  the  staminate  and  pistillate  flowers  are  not  on  the  same 
level,  consequently  the  pollen  floating  horizontally  on  the  wind 
is  not  likely  to  fall  upon  any  of  the  pistillate  flowers  of  the  same 
twig.     If  the  flowers  are  hermaphrodite  they  are  also  usually 
dichogamous,  which  means  that  either  the  andrcecium  or  the 
gyncecium  matures  first  and  hence  the  pollen  cannot  fertilize 
an  ovule  of  the  same  flower. 

143.  Entomophilous    hermaphrodite    flowers    are    usually 
either  dichogamous,  dimorphic  or  else  by  movements  of  stamen 
and  pistil  a  result  is  brought  about  which  is  practically  equiva- 
lent to  that  attained  by  dichogamy.     In  the  case  of  dimorphic 
flowers  there  are  two  kinds  of  flowers  which  differ  with  respect 
to  the  length  of  the  style  and  stamens,  and  the  position  of  the 
stigma  in  one  form  of  flower  corresponds  to  the  position  of  the 
anther  in  the  other.     The  result  of  this  is  that  when  an  insect 
visits  the  flower  it  receives  pollen  on  that  part  of  its  body  with 
which  the  stigma  comes  in  contact  when  the  insect  visits  a 
flower  of  the  other  type. 

144.  Self-fertilization  is  also  known  to  occur  in  many  herma- 
phrodite flowers.     However,  it  takes  place,  usually,  only  after 
the  methods  for  securing  cross-fertilization  have  been  employed 
by  the  flower;  the  result  being  to  insure  fertilization  in  case 

5 


cross-fertilization  fails.  Autogamy  is  secured  in  a  great  variety 
of  ways.  Some  of  these  are;  by  movements  of  the  anthers, 
by  movements  of  the  stamens  or  style,  or  both,  by  changes 
in  length  of  stamens  or  style,  by  changes  in  the  corolla,  etc. 


FIG.  33. — The  inflorescence  of  the  dog- wood.  The  flowers  are  small  and 
greenish  and  occur  in  clusters.  Beneath  each  group  of  flowers  are  four  large 
white  bracts  which  take  the  place  of  the  petals  in  making  the  inflorescence 
conspicuous. 

145.  Some  plants  develop  seed  from  flowers  which  never  open. 
Such  plants  also  produce  blossoms  under  favorable  conditions 
and  the  cleistogamic  flowers  are  to  be  regarded  merely  as  a 
special  form  of  autogamy.  The  corolla  is  reduced  and 
other  parts  of  the  cleistogamic  flower  may  differ  from  the 
corresponding  parts  of  the  blossoming  flower.  < 


SELECTION    OF   POLLEN  67 

146.  It  undoubtedly  often  occurs  that  various  kinds  of  pollen 
fall  on  the  same  stigma.  This  is  quite  likely  to  be  the  case 
with  anemophilous  pollen,  but  occurs  less  frequently  in  ento- 
mophilous  flowers  because  many  species  of  insects  confine  their 
attention  to  one  or  a  few  species  of  flowers;  and  also  many 
species  of  flowers  are  visited  by  only  one  or  a  few  species  of 
insects. 

1 4 7. "When  pollen  from  a  distantly  related  plant  falls  on  the 
stigma  of  a  flower  no  fertilization  occurs.  If  the  pollen  comes 
from  a  nearly  allied  plant,  however,  fertilization  may  take 
place  and  the  resulting  offspring  will  be  a  hybrid.  But  normally 
only  pollen  coming  from  a  flower  of  the  same  species  is  effica- 
cious in  producing  fertilization.  If  several  kinds  of  pollen  fall 
on  the  same  stigma  at  about  the  same  time  there  may,  therefore, 
be  a  selection  of  the  kind  proper  to  the  plant.  As  between 
pollen  from  the  same  flower  and  pollen  from  another  flower  of 
the  same  species  it  is  quite  probable  that  there  may,  also,  be 
a  selection  in  favor  of  that  yielding  cross-fertilization.  This  is 
known  to  be  the  case  in  certain  plants  and,  by  analogy,  it  may 
occur  in  others. 

The  Seed 

148.  Fertilization  accomplished,  the  development  of  the  seed 
begins.     The  embryo  itself  is  developed  from  the  germ  nucleus 
which  results  from  the  fusion  of  the  fertilizing  pollen  nucleus 
and  the  egg  nucleus  of  the  ovule.     But  the  germ  nucleus  is 
only  a  small  part  of  the  ovule.     The  other  parts  also  grow  as 
the  embryo  develops,  and  form  the  masses  of  reserve  food  and 
the  seed  coats. 

The  Fruit 

149.  While  the  ovules  in  the  ovary  are  developing  into  seeds, 
changes  are  also  taking  place  in  adjacent  parts  of  the  flower 
—changes   which   would   not  occur   if    the   ovules   failed   of 


68  PLANTS 

fertilization.  Sometimes  all  the  tissues  of  a  flower  cluster  are 
involved,  more  frequently  the  receptacle  or  calyx,  but  always 
the  ovary.  The  structures  become  enlarged  and  fleshy  or 
indurated  and  modified  in  various  other  ways.  The  resulting 
structure  or  organ  is  called  a  fruit  and  its  function  always  has 
relation  to  the  function  of  the  seed. 

150.  With  the  ripening  of  the  seed  the  plant  seems  to  have 
fulfilled  the  object  of  its  existence  and  soon  dies,  or  if  it  is  not 
an  annual  it  becomes  dormant  until  the  following  season,  when 
another  period  of  growth  is  closed  by  the  ripening  of  the  fruit. 
Occasionally  the  fruit  is  not  matured  until  the  following  season, 
in  which  case  the  activity  of  the  plant  during  the  first  season 
is  devoted  to  the  storing  up  of  reserve  food,  which  is  then 
used    in    the    development   of   the   fruit   during   the   second 
season. 

151.  The  distinction  between  seed  and  fruit,  then,  lies  in 
this,  that  the  seed  is  only  that  part  which  develops  from  the 
ovule,  while  the  fruit  includes  the  seed  and  consists  besides  of 
the  modified  ovary  and  frequently  other  adjacent  parts  of  the 
flower,  which  finally  together  constitute  the  seed-containing 
organ  of  the  plant. 

152.  Simple  fruits  are  either  fleshy  or  dry,  and  the  latter 
are  either  indehiscent  or  dehiscent,  hence  the  following  classes 
of  fruits  are  recognized: 

153.  A  berry  is  a  fleshy  fruit  composed  wholly  of  the  peri- 
carp, or  of  the  pericarp  and  the  adherent  calyx-tube. 

154.  The  drupe,  or  stone  fruit,  is  also  a  fleshy  pericarp,  the 
inner  layer  of  which  is  stony. 

155.  The  pome  is  a  fleshy  fruit  derived  from  the  concave 
receptacle  which  encloses  the  dry  papery  pericarp. 

156.  An  achene  is  a  dry  indehiscent  fruit  derived  from  a 
simple  pistil  and  containing  only  a  single  seed. 

157.  A  caryopsis  resembles  an  achene,  but  has  the  seed  coats 
intimately  united  with  the  walls  of  the  ovary. 


FRUITS  69 

158.  The  nut  also  resembles  an  achene,  but  is  derived  from 
a  pericarp  consisting  of  more  than  one  carpel. 

159.  A  samara  is  an  indehiscent  fruit  with  winged  appendages. 

1 60.  A  schizocarp  is  a  compound  fruit  which  splits  when 
ripe  into  two  or  more  parts,  each  resembling  an  achene. 

161.  A  follicle  is  a  dry  fruit  derived  from  a  simple  pistil  and 
opens  when  ripe  by  splitting  down  one  side. 

162.  A  legume,  or  pod,  is  like  the  follicle,  except  that  it  splits 
down  both  sides  of  the  carpel. 

163.  A  capsule  is  a  dry,  dehiscent  fruit  derived  from  a  com- 
pound pistil  and  opens  by  splitting  down  the  side,  by  separating 
a  lid  from  the  top,  by  opening  of  small  pores  or  otherwise. 

164.  Aggregate  fruits  are  those  which  are  made  up  of  the 
numerous  distinct  carpels  of  a  single  flower  adhering  together 
to  form  a  single  mass,   or  sometimes  held  together  by  the 
receptacle. 

165.  Multiple  fruits  are  composed  of  the  combined  carpels 
and  coherent  parts  of  a  number  of  flowers  held  together  by 
the  common   receptacle.     The   common   receptacle   may   be 
either  convex  or  concave,  in  the  latter   case   enclosing    the 
carpels. 

Seed  Distribution 

1 66.  All  the  elaborate  adaptations  of  the  plant  contribute 
directly  to  the  one  end — that  seed  may  be  produced  from  which 
a  new  generation  of  the  species  may  proceed.     However,  to 
produce  mature  seed  is  not  of  itself  a  guarantee  that  from  that 
seed  a  new  plant  will  spring.     The  seed  must  be  brought  to  a 
spot  where  the  conditions  are  favorable  for  its  germination  and 
development.     But  because  of  constantly  changing  conditions 
this  is  frequently  not  the  case  at  the  place  where  the  seed  was 
brought  to  maturity.     The  seeds  must  be  scattered  abroad  in 
order  that  some  may  by  chance  fall  upon  good  ground. 

167.  Some  seeds  are  so  small  and  light  that  they  are  readily 


70  PLANTS 

carried  by  the  wind  to  great  distances.  Larger  seeds  in  many 
cases  have  special  contrivances  in  the  form  of  sails,  parachutes, 
or  feathery  or  hair-like  appendages  which  offer  such  a  large 
surface  to  the  wind  that  they  may  also  be  carried  by  it  in  spite 
of  their  larger  size.  Seeds  swallowed  by  animals  are  frequently 
not  digested  and  may  be  carried  abroad  by  this  agency. 

1 68.  But  aside  from  the  protection  which  the  fruit  tissues  may 
give  the  seed  in  some  instances,  it  is  often  the  function  of  the 


FIG.  34. — A  leaf  of  Bryophyllum  developing  new  plantlets  by  budding  at  the 

edge  of  the  leaf. 

fruit  to  provide  for  the  dissemination  of  the  seed.  This  object 
may  be  accomplished  in  an  endless  variety  of  ways;  sometimes 
by  mechanically  scattering  the  seed  when  the  fruit  opens; 
sometimes  by  the  development  of  hold-fast  organs  which  cause 
the  fruit  to  cling  to  passing  animals;  or  again  by  means  of  para- 
chutes and  sails  by  which  the  fruit  is  carried  on  the  wind;  or  by 
floats  on  which  the  fruit  drifts  with  the  current  of  water.  Edi- 
ble fruits  of  all  sorts  are  carried  by  animals  from  place  to  place 
and  the  seeds  scattered  in  this  way.  In  some  cases  the  larger 
part  of  the  plant  is  concerned  in  the  process  of  scattering  seed. 


ASEXUAL   REPRODUCTION  71 

169.  It  is  no  t  in  all  cases  necessary  that  a  seed  should  be  formed 
in  order  that  a  new  plant  may  be  developed.     Many  peren- 
nials also  multiply  by  a  process  called  budding,  which  consists 
essentially  of  the  development  from  some  part  of  the  parent 
stock  of  a  shoot  which  ultimately  becomes  an  independent 
plant.     The  shoot  may  spring  from  the  roots,  from  underground 
stems,  from  runners  or  branches  where  they  touch  the  sub- 
stratum or  even  from  leaves.     Some  species  belonging  to  the 
group  of  seed-bearing  plants  have  adopted  this  method  of  repro- 
duction almost  to  the  exclusion  of  the  formation  of  seeds. 

Classes  of  Plants 

170.  All  the  so-called  flowering  plants  have  one  character- 
istic in  common,  which  is  the  formation  of  a  reproductive  body, 
the  seed,  developed  from  the  ovule  after  fertilization,  and  con- 
sisting essentially  of  an  embryo  enclosed  in  a  protective  seed 
coat.     This  group  of  plants  is  called  Spermatophytes  and  con- 
sists of  two  divisions,  the  Angiosperms  and  the  Gymnosperms. 

171.  The  Angiosperms  are  those  seed-bearing  plants  in  which 
the  ovules  are  enclosed  in  the  cavity  of  an  ovary.     Of  these 
there  are  two  classes,  the  Monocotyledons  and  the  Dicotyle- 
dons.    The  difference  between   these  two    classes  is   shown 
in  the  following  table: 

Angiosperms 

DICOTYLEDONS  MONOCOTYLEDONS 

1.  Two  seed  leaves.  i.  One  seed  leaf . 

2.  Leaves  netted  veined  and  2.  Leaves    parallel    veined    with 
with  broken  margin.  ^margin  entire. 

3.  Parts  of  flowers  in  45  or  55.  3.  Parts  of  flowers  in  35. 

4.  Vascular    bundles    of    the  4.  Vascular  bundles  of  the  stem 
stem  in  a  single  circle  form-  scattered  and  no  distinction  of 
ing  two  concentric  cylin-  wood  and  bark. 

ders  of  wood  and  bark. 
Either  of  the  characters,  2,  3,  or  4,  may  in  some  cases  fail  to  apply. 


72  PLANTS 

172.  The  grasses,  sedges,  lilies,  palms  and  orchids  are  the 
most  important  groups  of  the  Monocotyledons.  The  Dicoty- 
ledons include  most  of  the  remaining  seed-bearing  plants  except 
the  "evergreens." 


FIG.  35. — Inflorescences  of   the  pine,     i,  Terminal  twig;    2,  ovulate  cone;   3, 
staminate  cone;  4,  two-year-old  cone. 

Gymnosperms 

173.  The  Gymnosperms  are  distinguished  from  the  Angio- 
sperms  by  the  fact  that  the  ovules  are  not  enclosed  by  the  walls 
of  an  ovary,  but  are  simply  covered  by  a  scale.     To  this  group 
belong  the  cone-bearing  "evergreens;"  as  e.  g.,  the  pine,  cedar, 
yew,  larch,  and  spruce. 

Cryptogams 

174.  Not  all  plants  produce  seed.     There  is  a  great  variety 
of  organisms  which  are  not  included  in  the  groups  so  far  consid- 


CLASSES   OF  PLANTS  73 

ered;  e.  g.,  ferns,  mosses,  sea- weeds,  toadstools,  molds,  etc. 
These  are  all  grouped  together  under  the  name  of  Cryptogams, 
but  it  is  not  thereby  meant  to  indicate  that  there  is  a  close 
relationship  between  the  various  members  of  the  group.  It 
signifies  only  that  the  members  of  which  it  is  composed  do  not 
bear  seed.  As  a  whole  the  Spermatophytes  are  much  more 
complex  and  for  certain  reasons  are  regarded  as  of  a  higher  order 
than  the  Cryptogams. 

175.  Of    the    Cryptogams,    the   highest    class — those   most 
nearly  resembling  the  Spermatophytes — are  the  Pteridophytes, 
including  the  ferns  and  their  allies.     Most  of  these  have  an 
underground  stem  (rootstock  or  rhizome)  with  a  system  of  true 
roots  and  a  series  of  leaves  held  aloft  on  long  petioles  or  stipes. 
The  microscopic  structure  of  the  organs,  too,  resembles  in  a 
general  way  that  of  similar  organs  in  the  higher  plants.     The 
common  ferns,  the  scouring-rushes  and  the  club-mosses  are 
familiar  examples  of  this  group. 

176.  Next  in  order  below  the  Pteridophytes  come  the  Bryo- 
phytes,   to   which  group  belong  the   mosses   and  liverworts. 
These  plants  are  all  small.     The  moss  plant  consists  of  a  slender 
stem,  with  scale-like  leaves,  but  no  true  roots,  and  there  are  no 
well-developed  vascular  bundles  in  the  stem.     In  liverworts 
there  is  usually  no  distinction  of  stem  and  leaf.     The  body  of 
the  plant  consists  simply  of  a  flat  expanse  of  green  tissue.     In 
place  of  roots  the  mosses  and  liverworts  have  organs  which 
resemble  root  hairs,  and  are  called  rhizoids. 

177.  All  plants  not  included  in  the  foregoing  groups  are 
classed  together  as  Thallophytes — a  large  and  heterogeneous 
group  which  comprises  all  the  lower  or  simpler  plants.     The 
body  of  a  Thallophyte  is  never  differentiated  into  root,  stem 
and  leaves,  as  is  usually  the  case  in  the  higher  groups,  and  there 
are  more  exact  distinctions  to  be  observed  in  the  methods  of 
reproduction.     The  Thallophytes  are  divisible  into  two  very 
distinct  groups,  algae  and  fungi,  which  are  distinguished  by  the 


74  PLANTS 

presence  of  chlorophyll  in  the  former  group  and  its  total  absence 
in  the  latter.  Most  of  them  are  small,  many  are  microscopic  in 
size,  but  there  are  a  few  marine  algae  which  are  extremely  large. 

178.  The  algae  are  found  either  in  the  water  or  else  in  moist 
places,  for  they  have  no  elaborate  protective  structures  which 
would  prevent  desiccation.     There  are  many  kinds  which  con- 
sist of  only  a  single  cell,  others  of  similar  cells  arranged  in  rows 
or  filaments.     In  others,  again,  the  cells  are  arranged  in  sheets 
or  masses  having  more  or  less  definite  forms.     One  group  of 
marine  algae  in  which  the  structure  is  rather  complex  is  charac- 
terized by  a  reddish  color,  due  to  the  presence  of  a  red  pigment 
in  the  protoplasm,  which  to  some  extent  obscures  the  green  of 
the  chlorophyll.     Another  group  of  marine  algae,  simpler  in 
structure,  is  similarly  characterized  by  a  yellow  pigment  which 
gives  the  plant  a  brownish  color.     A  small  group  of  extremely 
simple  filamentous  or  unicellular  algae  is  characterized  by  a 
blue-green  color  due  to  the  presence  of  a  blue  pigment.     There 
are  many  algae,  however,  which  are  neither  red,  brown  nor 
blue-green,  but  have  the  yellowish-green  color  characteristic  of 
chlorophyll.     These  vary  in  complexity  of  structure  from  the 
simplest  to  the  most  complex.     The  blue-green  and  the  green 
algae  comprise  both  marine  and  fresh  water  forms. 

179.  The  fungi  vary  as  greatly  in  regard  to  complexity  of 
structure  as  do  the  algae  and  may  be  regarded  as  a  parallel 
series,  differing  chiefly  from  the  algae  in  those  points  which  are 
dependent  on  the  presence  of   chlorophyll.     Since    they    are 
destitute  of  chlorophyll,  the  fungi  (excepting  perhaps  some  of 
the  lowest  forms)  cannot  assimilate  carbon  dioxide  and  con- 
sequently are  either  saprophytic,  i.  e.,  nourished  upon  waste 
organic  matter,  or  parasite,  i.  e.,  nourished  upon  the  tissues  of 
other  living  organisms.     Some  of  the  most  familiar  of  the 
higher  fungi  are  the  toadstools,  mushrooms,  shelf-fungi,  puff- 
balls,  smuts  and  rusts  of  grasses,  "  cedar- apple,"  ergot,  black- 
knot  of  plum  trees,  mildews,  molds,  yeast,  etc. 


CLASSES    OF   PLANTS  75 

1 80.  The  lowest  fungi  are  the  bacteria,  a  large  and  impor- 
tant group,  though  made  up  of  the  simplest  and  minutest  of 
all  organisms.     The  bacteria  are  minute  unicellular  or  fila- 
mentous organisms,  so  simple  in  structure  that  the  cell  con- 
stituting an  individual  seems  to  be  devoid  of  even  the  nucleus. 
To  this  group  belong  the  germs  of  many  diseases,  and  the  active 
agents  in  various  processes,  such  as  putrefaction  and  decay, 
souring  of  milk,  acid  and  vinous  fermentations,  etc. 

181.  Cryptogams  reproduce  by  means  of  spores  instead  of 
by  seeds.     Spores  are  single  cells  specially  set  apart  by  the  plant 
for  the  purpose  of  reproduction.     In  some  cases  they  are  formed 
by  the  union  of  two  elements,  as  in  the  process  of  fertilization 
in  Spermatophytes.     Another  kind  of  spore  is  formed  merely 
by  the  separation  from  some  part  of  the  parent  plant  of  a 
single  cell,  which  has  the  power  of  developing  a  new  plant  with- 
out fertilization.     Some  of  the  lowest,  simplest  Cryptogams, 
consisting  of  a  single  cell,  multiply  merely  by  the  division  of  the 
cell  into  equal  halves  (fission). 

Ecology 

182.  In  our  study  of  the  development,  form,  structure  and 
life  processes  of  a  plant  we  have  confined  our  attention  almost 
entirely  to  the  kinds  of  plants  with  which  we  have  been  most 
familiar,  i.  e.,  such  as  grow  in  soils  that  are  at  least  moderately 
productive  to  the  agriculturist  and  in  climates  which  are  the 
most  habitable  to  man,  neither  extremely  cold  nor  hot,  nor  ex- 
tremely wet  or  dry.     And  besides,  we  have  limited  our  study 
to  the  independent,  chlorophyll-bearing   plants.     In  these  we 
have  seen  with  regard  to  every  feature  of  the  plant's  organiza- 
tion a  remarkable  adjustment  to  its  external  conditions  of 
existence,  or,  in  other  words,  adaptation  to  environment.     This 
has  been  so  apparent  at  every  turn  that  one  might  well  regard 
it  as  a  law  of  nature.     However,  if  there  be  such  a  law,  it  must 


76 


PLANTS 


apply  to  all  plants  in  all  circumstances  under  which  they  are 
found  to  thrive,  although  the  environment  may  be  very  different 
from  that  which  is  normal  to  the  plants  we  have  been  consider- 
ing. In  order,  then,  to  test  the  validity  of  this  law,  let  us  ex- 
amine the  flora  of  localities  which  present  conditions  different 
from  those  we  have  already  considered, 


FIG.  36. — A  tree  deformed  by  the  action  of  the  wind  and  salt  spray.  The 
buds  are  continually  killed  on  the  windward  side.  Coast  of  North  Carolina. 
A  similar  effect  is  produced  by  the  combined  action  of  wind  and  cold,  as  on  high 
mountain  summits. 

Water 

183.  With  regard  to  conditions  of  moisture,  plants  have  been 
grouped    into    three    classes,    mesophytes,    hydrophytes    and 
xerophytes.     Mesophytes  are  the  plants  which  grow  normally 
under  conditions  of  moderate  supply  of  moisture,  and  hence 
include  all  those  which  we  have  heretofore  been  studying. 
Hydrophytes  are  plants  which  grow  in  the  water,  or,  at  least, 
in  very  wet  soils.     Xerophytes  are  the  plants  peculiar  to  arid 
regions. 

184.  Among  hydrophytes  we  may  have,  first,  those  plants 


ECOLOGY  77 

which  grow  entirely  submersed  either  in  the  sea,  in  fresh  water 
streams  or  in  quiet  ponds.  The  most  striking  peculiarities 
common  to  plants  living  under  such  conditions  is  the  almost 
complete  absence  of  mechanical  supporting  tissue.  Almost 
without  exception,  submersed  aquatics  are  not  rigid  enough 
to  support  their  own  weight  when  taken  from  the  water. 
Obviously  the  buoyancy  of  the  water  makes  such  a  highly 
developed  supporting  system  superfluous. 

185.  Some  aquatics  utilize  the  buoyancy  of  the  water  for 
support    by    specialized   bladder-like   floats    which    represent 
modified  leaf  blades,  petioles,  or  other  organs.     Very  generally, 
also,  the  tissues  of  such  plants  contain  extensive  systems  of 
passages  filled  with  air.     These  serve  not  only  to   aerate   the 
tissues,  but  at  the  same  time  act  as  floats. 

1 86.  Other  characters  common  to  plants  of  this  class  are  the 
undeveloped  condition  of  the  root  system,  which  usually  serves 
only  as  a  hold-fast,  and  the  absence  of  root  hairs.     The  absorp- 
tion of  water  is  carried  on  chiefly  by  the  epidermis  of  the  stem 
and  leaves.     For  the  epidermis,  not  being  exposed  to  the  dry 
air,  is  not  cutinized  and,  consequently,  is  in  condition  to  serve 
the  function  of  water  absorption. 

187.  The  leaves  of  submersed   aquatics  are  commonly  very 
narrow  or  finely  divided.     This  offers  several  advantages  under 
the  conditions;   the  ratio  of  absorbing  surface  is  increased, 
mutual  shading  lessened  and  there  is  less  resistance  to  currents 
of  water  which  would  tend  to  dismember  the  plant.     Besides, 
in  a  submersed  plant,  there  would  be  no  apparent  advantage 
offered  by  a  broad  leaf  over  an  equal  expanse  of  narrow  leaves. 

1 88.  Plant  surfaces  continually  in  contact  with  water  have 
no  stomata,  hence  the  gases  absorbed  in  the  case  of  submersed 
aquatics  are  taken  from  the  water  by  osmose. 

189.  In  marked  contrast  with  the  finely  divided  leaves  of 
submersed  plants  are  the  broad  leaves  of  the  floating  aquatics. 
The  under  surface  of  leaves  of  this  type  is  destitute  of  stomata, 


PLANTS 


but  the  upper,  exposed  surface,  has  the  stomata  and  an  epider- 
mis like  that  of  the  mesophytes.  These  plants  grow  only  in 
quiet,  shallow  water.  They^are  firmly  rooted  in  the  mud, 


FIG.  37. — Salicornia  ambigua,  a  xerophytic  plant  found  in  salt  marshes  and 
sandy  beaches  of  the  Atlantic  sea-board.     The  leaves  are  rudimentary.      Xi/2. 

from  which  the  long  stout  petioles  rise  at  an  angle  to  the  surface, 
where  the  broad  leaf  blades  spread  out  in  a  single  plane.  These 
conditions  allow  the  leaf  to  rise  and  fall  with  every  change  in 


ECOLOGY  79 

the  level  of  the  surface.  There  can  be  no  question  of  mutual 
shading  and  none  of  the  considerations  which  gave  advantage 
to  the  form  of  the  submersed  leaf  can  here  apply. 


FIG.  38. — A  xerophytic  habit  of  the  prickly  lettuce,  Lactuca   scariola.     View 
as  seen  from  the  east. 

190.  A  few  plants  are  capable  of  growing  either  entirely 
under  water  or  with  at  least  some  of  the  leaves  entirely  above 


80  PLANTS 

water.  In  these  the  effect  of  the  water  on  the  form  of  the  leaf  is 
clearly  shown.  Both  kinds  of  leaf,  the  narrow  ones  below  the  sur- 
face and  the  broad  ones  above,  may  be  found  on  the  same  plant. 


FIG.  39. — Lactuca  as  seen  from  the  south.  This  and  the  preceding  figure 
show  the  leaves  twisted  into  the  vertical  plane  and  bent  toward  the  plane  of 
the  meridian. 

191.  Along  the  border  of  quiet  waters  another  type  of  hydro- 
phyte is  to  be  found.  The  plants  of  this  type  stand  in  shallow 


ECOLOGY  8 I 

water,  firmly  rooted  in  the  mud,  but  are  erect,  self-supporting 
and  rise  tall  and  slender,  high  above  the  surface  of  the  water. 
The  special  adaptation  here  is  in  the  height  of  the  plant,  which 
permits  considerable  change  in  level  of  the  water  surface  with- 
out drowning  the  plant. 

192.  Marsh  and  swamp  plants  do  not  differ  much  from  the 
mesophytes   in   structure,    but   nevertheless    the    continually 
saturated  soil  and  other  conditions  which  obtain  in  such  locali- 
ties,  are  sufficiently  different  from  mesophyte  conditions,  on 
the  one  hand,  and  true  hydrophyte  conditions  on  the  other,  to 
give  the  floras  of  swamps  and  marshes  a  character  of  their  own. 

193.  Comparing  xerophytic  plants  with  hydrophytes  we  find 
that  in  a  number  of  particulars  they  present  the  opposite  ex- 
tremes of  structures.     Thus  the  epidermis  of  xerophytes  is 
extremely  well  developed,  often  consisting  of  several  layers  of 
cells  and  provided  with  a  very  thick  cuticular  wall  on  the  sur- 
face.    Stomata  are  less  numerous.     The  plant  as  a  whole  is 
more  compact,  thus  reducing  the  ratio  of  surface  to  volume. 
All  these  peculiarities  result  in  decreased  loss  of  water  by 
transpiration  and  evaporation  and  are  clearly  an  adaptation  to 
scanty  water  supply.     The  massive  form  of  these  plants  also 
affords  space  for  the  storage  of  water  obtained  from  occasional 
showers. 

194.  Under  semi-arid  conditions  there  is  sometimes  another 
device  employed  for  preventing  the  excessive  loss  of  water, 
namely,   the  vertical  or  meridional  position  assumed  by  the 
leaves  or  leaf -like  organs  (phyllodes)  and  the  consequent  tem- 
pering of  the  force  of  the  sun's  rays. 

195.  In   those  regions  of   the  earth's   surface  which  have 
alternately  wet  and  dry  seasons  the  vegetation  also  presents 
alternately  mesophytic  and  xerophytic  characters.     This  does 
not  mean  merely  that  at  one  season  mesophytes  are  prominent 
and  at  another  the  xerophytes,  but  even  the  same  plant  alters 
in  character  with  the  seasons. 

6 


82  PLANTS 

Temperature 

196.  There  are  certain  parts  of  the  earth's  surface  which  are 
always   destitute  of  vegetation  because  of  the  fact  that   the 


FIG.  40. — A  small  fern,  Polypodium,  which  grows  as  an  epiphyte  on  the  bark  of 
trees.     See  next  figure.     XL 

surface  waters  are  always  frozen,  a  condition  which  renders 
vegetable  life  impossible.     On  the  other  hand,  in  the  hottest 


ECOLOGY 


parts  of  the  earth  vegetation  flourishes,  provided  there  is  a 
sufficient  supply  of  moisture.  In  fact,  it  is  in  equatorial  regions 
that  vegetation  grows  most  luxuriantly. 

197.  Between  the  two  extremes  of  latitude,  with  the  corre- 
sponding extremes  of  cold,  and  heat  and  absence  and  profusion 
of  vegetation,  there  are  regions  which  present  an  alternation  of 
conditions  with  respect  to  temperature,  from  winter  to  summer. 


FIG.  41. — Same  as  the  preceding  figure  but  photographed  on  the  preceding 
day.  The  plant  has  the  xerophytic  habit  of  curling  up  during  dry  weather  as  in 
this  figure.  In  wet  weather  the  leaves  expand. 


The  change  of  seasons  permits  only  of  intermittent  periods  of 
growth,  and  this  has  affected  most  of  the  species  indigenous 
to  such  regions  to  such  an  extent  that  the  life  processes  succeed 
each  other  in  a  rhythmical  manner,  even  though  the  conditions 
are  temporarily  altered.  The  lower  orders  of  plants  respond 
more  directly  to  actual  changes  of  the  conditions,  and  they  vary 
in  character  directly  as  the  conditions  vary,  but  the  higher 


84  PLANTS 

i 

forms  show  a  decided  tendency  to  undergo  their  usual  series 
of  life  processes  and  accompanying  change  of  character,  even 
though  the  seasonal  changes  of  conditions  fail  to  occur  at  the 
appropriate  time.  For  example,  the  tropical  plants  present  an ; 
expanse  of  leaf  surface  throughout  the  year,  although  the  older ; 
leaves  are  continually  falling,  because  new  ones  are  as  constantly 
developing.  The  deciduous  perennials  are  characteristic  of 
temperate  latitudes,  where  there  is  alternately  winter  and  sum- 
mer. In  the  latter  case  there  is  evidently  a  relation  between 
the  fall  of  the  leaf  and  the  seasons.  But  the  leaves  do  not  fall 
only  after  they  have  been  killed  by  a  frost.  Rather,  they  die, 
and  physiological  connection  with  the  plant  body  is  cut  off 
before  the  time  for  serious  frosts  arrives.  Otherwise  not  only 
would  the  leaves  be  killed,  but  the  plant  itself  might  suffer 
serious  injury. 

198.  For  the  plant  to  retain  its  leaves  during  the  snows  of 
winter  would  also  expose  it  to  the  danger  of  being  overloaded 
and  crushed  by  sheer  weight  of  the  snow. 

199.  Another  example  of  this  principle  of  the  independence 
of  the  plant  of  the  direct  conditions  of  its  environment  is  found 
in  the  period  of  rest  required  by  seeds  and  other  reproductive 
bodies,  such  as  bulbs,  which  normally  remain  quiescent  during 
the  winter  and  resume  their  growth  with  the  recurrence  of  the 
warmth  of  spring.     But  the  rest  is  taken  whether  or  not  the  win- 
ter conditions  supervene. 

200.  Such  adaptations  of  the  plant  are  not  responses  to 
changing  external  conditions.     The  plant  undergoes  changes 
which  anticipate  the  corresponding  changes  in  the  conditions. 
Such  adaptations  must  be  regarded  as  habitual  responses. 

20 1.  Some    other    adaptations   of   the   plants   to   rigorous 
climates  are,  for  example,  such  modifications  of  the  leafy  shoot 
as  the  rosette  and  the  creeper.     By  hugging  the  earth  plants 
of  this  type  avoid  the  great  exposure  to  cold  which  a  freer 
method  of  growth  would  entail. 


ECOLOGY 


202.  Subterranean  stems  and  reserve  food  stores  generally 
ire  devices  for  tiding  over  unfavorable  seasons  and  permitting 
the  plant  to  make  the  best  of  a  short  growing  season.  The 


FIG.  42. — Cassia,  the  wild  sensitive  pea. 

mnual  and  biennial  plant  habits  are  also  evidently  adaptations 
:o  seasonal  changes,  whether  of  temperature  or  moisture. 

203.  The  usual  fall  of  temperature  at  night,  which  is  often 
rery  considerable,  is  a  condition  to  which  some  plants  apparently 


86  PLANTS 

show  a  very  special  response.  The  usual  day-time  disposition 
of  the  leaves  is  such  as  would  at  night  result  in  the  greatest 
loss  of  heat  by  radiation.  The  leaves  of  many  plants  droop  at 
night  and  thereby  come  into  a  position  which  greatly  reduces 
the  loss  of  heat  by  radiation. 

Latitude  and  Altitude 

204.  The  traveler  in  passing  from  the  equator  to  the  latitudes 
of  perpetual  snow  in  polar  regions  observes  a  gradual  change  in 
the  character  of  the  vegetation  from  the  most  luxuriant  evergreen 
tropical  forests  to  the  scanty  herbage  of  those  high  latitudes 
where  during  the  few  weeks  of  the  brief  summer,  while  the  ground 
is  bared  of  snow,  a  few  specially  hardy  mosses,  a  few  rapidly 
maturing  annual  and  biennial  herbs  and  still  fewer  shrubby 
perennials    succeed   in  bringing  their  fruit  to  maturity.     So 
also  in  ascending  mountain  slopes  from  the  sea-level  at  the 
equator  to  the  snow  line  on  the  higher  peaks  a  similar  series 
of  changes  in  the  character  of  the  vegetation  occurs  with  the 
degrees  of  altitude.     In  middle  altitudes,  as  in  middle  latitudes, 
there  is  an  intermediate  condition  of  vegetation  characterized 
by  the  grasses  of  the  prairies  and  the  deciduous  perennials  and 
coniferous  evergreens  of  the  forests. 

Light 

205.  The  adaptations  of  green  plants   to  light  conditions 
has  been  discussed  at  considerable  length  with  reference  to  the 
disposition  of  the  leaves.     It  remains  to  show  that  the  adapta- 
tion extends  also  to  the  formation  of  palisade  tissue  and  the 
arrangement  of  the  chloroplasts  within  the  cells  of  themesophyll. 
In  order  to  determine  whether  the  palisade  tissue  is  the  result 
of  a  response  to  light  stimulus,  the  following  experiment  was 
performed.     A  developing  leaf  was  artificially  inverted  so  that 


ECOLOGY 


what  should  have  been  the  underside  was  brought  uppermost 
and  facing  the  sun.  The  result  was  that  the  palisade  tissue  was 
developed  on  the  side  toward  the  light,  that  is,  on  the  mor- 
phological underside. 


FIG.  43. — Leaves  of  the  sensitive  pea  closed.     In  this  case  the  leaves  close  when 
touched.     No  explanation  for  this  habit  is  known. 

206.  The  position  taken  by  the  chloroplast  in  the  cells  is 
also  determined  by  the  intensity  of  the  light.     In  bright  light 


88  PLANTS 

i 

they  are  found  to  be  crowded  on  the  vertical  walls  of  the  cells, 
while  in  subdued  light  they  are  ranged  on  the  horizontal 
walls,  thus  exposing  themselves  broadside  to  the  light.  In  this 
way  the  chloroplasts  to  a  considerable  degree  control  the 
light  relation  of  the  plant. 

207.  Plants  might  be  classified  with  reference  to  the  light 
conditions  of  their  habitat.     Many  species  grow  only  in  shaded 
situations,  while  others  seek  the  brightest  light.     Under  the 
vertical  rays  of  a  tropical  sun  the  light  is  much  more  intense  than 
it   is   in   higher   latitudes.     Consequently   it   penetrates    the 
foliage  of  the  taller  forest  vegetation  with  sufficient  intensity 
to  permit  also  of  a  vigorous  undergrowth.     The  result  is  that 
other  things  being  equal  the  intensity  of  the  light  in  the  tropics 
permits  a  denser  growth  of  vegetation  than  could  exist  in  higher 
latitudes. 

208.  The  deep  sea  is  known  to  be  practically  destitute  of 
vegetation,  although  the  conditions,  except  for  darkness,  are 
probably  favorable.     Along  shore  and  on  the  surface  of  the  sea 
there  is  an  abundance  of  green  and  brown  sea- weed  vegetation, 
and  farther  down,  on  comparatively  shallow  bottom,  the  red 
sea-weeds  are  found.     It  has  been  suggested  that  the  red  and 
brown  pigments  of  the  red  and  brown  sea- weeds  have  some 
significance  with  reference  to  the  light  relation. 

Soil 

209.  Every  farmer  is  familiar  with  the  fact  that  the  dis- 
tribution of  plants  is  largely  determined  by  the  nature  of  the 
mineral  constituents  of  the  soil.     Thus  limestone  regions  are 
better  adapted  for  the  cultivation  of  certain  crops  than  are  soils 
derived  from  sandstones  or  shales.     So  also  other  plants  do  well 
only  on  sands  or  clays.     Analysis  of  the  soils,  however,  shows 
that  all  the  mineral  substances  necessary  to  any  plant  are 
present  in  sufficient  quantities  in  any  soil.     It  is  also  known  that 


ECOLOGY  89 

plants  differ  greatly  in  regard  to  their  ability  to  thrive  on  soil 
containing  little  organic  matter,  and  it  is  probable  that  this  is 
really  the  determining  cause  of  this  apparent  soil  relation. 
Soils  of  unlike  mineral  constitution  do  not  retain  the  organic 
matter  to  the  same  degree  and  it  is  therefore  probably  the 
amount  of  humus  present  in  the  soil  that  determines  its  adapta- 
bility to  any  given  plant. 

210.  Occasionally  soils  are  too  unstable  to  permit  vegetation 
to  secure  a  foothold.     This  is  notably  true  of  the  shifting  sand 
dunes  of  many  windward  coasts  and  in  sandy  deserts.     There 
are  a  few  plants,  however,  which  are  enabled  to  maintain  their 
position  in  such  soil  by  virtue  of  rapid  and  deep  rooting  or, 
better  still,  by  means  of  stolons  or  runners  which  enable  the 
individual  stocks  to  cling  to  each  other  and  finally,  forming  a 
felted  carpet,  protect  the  sand  from  the  wind  and  hold  it  in 
place. 

Relation  of  Plants  to  Each  Other 

211.  We  have  heretofore  spoken  of  the  green  plants  as  being 
independent  in  the  sense  of  deriving  their  sustenance  directly 
from   inorganic   matter.     This   might   be   regarded   as   quite 
generally  true  wherever  the  plant  is  provided  with  soluble 
nitrogen  compounds.     However,  these  salts  are  by  no  means 
everywhere  present  in  the  soil,  and  under  such  circumstances 
green  plants  become  dependent  upon  certain  fungi,  as  we  shall 
presently  see. 

212.  Plants  may  be  dependent  upon  other  plants  in  a  great 
variety  of  ways,  and  in  varying  degrees.     The  climbers,  for 
example,  get  only  mechanical  support  from  their  stouter  neigh- 
bors.    Some  cling  to  their  support  by  means  of  aerial  rootlets, 
which  penetrate  the  outer  layers  of  the  bark  of  the  host.     Others 
spread  their  long  slender  tendrils,  which,  on  contact  with  a  solid 
object,  coil  around  it  and  then  draw  the  stem  of  the  plant  close 


QO  PLANTS 

to  the  support  by  the  contracting  spirals.  Tendrils  in  some 
instances  eiid  in  adhesive  discs  by  which  the  plant  is  enabled  to 
cling  to  a  smooth  plane  surface.  Lastly,  the  twining  climbers, 
swaying  their  growing  tips  in  a  spiral  around  the  axis  of  support, 
coil  themselves  bodily  about  their  host.  In  neither  of  these 
cases  does  the  climber  obtain  any  nourishment  from  the  host, 


FIG.  44. — The  trumpet  vine,  a  climber. 

although  the  latter  may  be  seriously  handicapped  or  even 
finally  destroyed  through  shading  by  its  vigorous  yet  dependent 
hanger-on. 

213.  Epiphytes   constitute   another   class   of   plants   which 
depend  upon  others  for  mechanical  support.     They  have  no 


ECOLOGY  91 

connection  with  the  soil  and  obtain  their  necessary  supply  of 
moisture  from  the  humid  atmosphere,  from  the  moist  bark  of 
the  host,  or  from  the  sponge  of  vegetable  detritus  which  accu- 
mulates about  the  base  of  the  plant.  The  epiphyte  by  its  habit 
merely  obtains  advantageous  exposure  to  light. 


FIG.  45. — An  epiphyte,  Tillandsia,  hanging  from  the  branches  of  trees, 
landsia  is  a  flowering  plant  but  is  erroneously  called  "gray  moss." 


Til- 


214.  Saprophytes  are  plants  found  growing  only  on  humus 
or  other  decaying  organic  matter.  They  are  of  great  impor- 
tance in  the  economy  of  nature  because  of  their  share  in  the 
process  of  decomposition  and  decay.  They  contain  no  chloro- 


PLANTS 


FIG.  46. — Puff-balls,  Lycoperdon,  a  saprophyte.     XL 


FIG.  47. — The  truffle,  Tuber  brumale;  a  saprophyte  which  grows  underground, 

X3/4- 


ECOLOGY 


93 


phyll,  have  no  power  of  photosynthesis  and  consequently  bear 
no  necessary  light  relation.  The  group  includes  chiefly  fungi, 
but  there  are  not  a  few  flowering  plants  which  have  degenerated 
to  the  condition  of  saprophytes. 

215.  What  has  been  said  of  saprophytes  might  be  repeated 
for  the  group  called  parasites;  excepting  this,  that  they  live  in 


FIG.  48. — Section  ol  a  lichen.  Near  the  upper  surface  are  groups  of  rounded 
cells  (shaded).  These  are  algal  cells,  arranged  in  groups  by  fission.  The  re- 
maining parts  are  formed  by  the  filaments  of  the  fungus.  (From  Sayre  after 
Sachs.) 

or  upon  the  tissues  of  living  organisms  and  the  result  of  their 
activity  is  called  disease.  Most  infectious  diseases  of  both 
animals  and  plants  are  to  be  ascribed  to  this  class  of  organisms. 
A  few  chlorophyll-bearing  plants  have  a  parasitic  habit  and 
hence  are  called  partial  parasites.  Facultative  parasites  may 
live  either  as  true  parasites  or  as  saprophytes,  while  obligate 
parasites  can  exist  only  as  parasites. 


94 


PLANTS 


2 1 6.  Symbionts  are  organisms  which  are  associated  with 
other  plants  or  animals  for  the  advantage  of  one  or  both,  but 
without  serious  detriment  to  either.  Some  forms  are  found 
normally  only  under  such  relationship.  Lichens  are  a  symbiotic 
combination  of  a  fungus  and  an  alga.  A  relation  of  this  kind 
also  exists  between  certain  Spermatophytes  and  some  of  the 


FIG.  49. — The  sun-dew,  Drosera.     One  leaf  is  shown,  with  the  glandular  hairs 
by  which  small  insects  are  caught.     X5/3- 

lower  fungi.  The  mycorhiza,  for  example,  consist  of  a  filamen- 
tous fungus  attached  to  the  roots  of  seed  plants,  for  which  they 
seem  to  serve  to  some  extent  the  office  of  absorption,  and  prob- 
ably receive  some  compensation  in  return. 

217.  The  most  important  of  this  class  of  plant  relationship 


ECOLOGY  95 

is  that  which  exists  between  bacteria  and  seed  plants.  Certain 
bacteria  live  in  the  soil  and  have  the  power  of  assimilating  the 
free  nitrogen  of  the  air,  of  breaking  up  ammonia  compounds,  or 
of  oxidizing  nitrites  into  nitrates  and  thus  bringing  the  nitrogen 
compounds  into  a  form  available  for  green  plants.  Such  nitro- 
gen bacteria  accumulate  in  masses  on  the  roots  of  leguminous 


FIG.  50.— The  trap  of  the  Venus  fly- trap,  half  closed.     X2. 

plants,  presumably  finding  there  conditions  favorable  to  their 
own  development  and  certainly  enabling  the  associated  seed 
plant  to  thrive  in  soil  which  would  otherwise  be  too  poor  in 
available  nitrogenous  compounds  to  support  the  plant. 

Carnivorous  Plants 

2 1 8.  A  considerable  variety  of  unrelated  plants  have  acquired 
the  power  in  one  form  or  other  of  capturing  small  animals  and 


96 


PLANTS 


by  a  process  analogous  to  digestion  and  absorption  securing 
in  this  way  the  requisite  nitrogenous  matter.  Such  carnivorous 
plants  are  found  in  soils  or  situations  which  are  in  an  unusual 
degree  devoid  of  nitrogenous  compounds  of  all  kinds. 

Physiographic  Relations 

219.  Finally  we  may  note  briefly  the  physiographic  relations 
of  plants.     From  geological  evidence  we  know  that  vegetation 


FIG.  51. — The  Venus  fly-trap  closed,  showing  interlocking  teeth. 

has  existed  upon  the  earth  for  vast  ages  and  has  undergone 
continual  changes  with  the  lapse  of  time.  Long  before  the 
advent  of  man,  vegetation  flourished  as  it  has  not  done  since, 
and  in  certain  regions  of  the  earth's  surface  the  remains  of 
those  early  forests  accumulated  to  such  an  extent  as  to  form 


ECOLOGY 


97 


FIG.  52. — Venus  fly-trap.     Teeth  unlocked  but  trap  still  closed.     The  dark 
shadow  is  cast  by  the  bodies  of  two  house  flies  caught  by  the  trap. 


FIG.  53. — Trumpets,  one  of  the  pitcher  plants  (Sarracenia  flava). 
7 


98  PLANTS 

the  many  deposits  of  coal,  covering  in  the  aggregate  thousands 
of  square  miles  of  the  earth's  surface  and  varying  in  thickness 
from  a  few  inches  to  hundreds  of  feet. 


FIG.  54. — Sarracenia  minor. 


220.  Practically  all  the  coal-forming  plants  have  long  since 
become  extinct.  They  belonged  chiefly  to  the  Cryptogams, 
and  very  few  of  the  higher  plants  existed  at  the  time.  The 


ECOLOGY 


99 


species  of  Spermatophytes  living  to-day  have  all  appeared  on 
the  earth  in  comparatively  recent  times. 

221.  Plant  remains,  therefore,  in  the  form  of  coal,  constitute 
a  very  considerable  part  of  the  earth's  crust  and  have  largely 
determined  the  modern  conditions  of  human  activity.  But 


FIG.  55. — Sarracenia  purpurea,  in  bloom.     In  this  species  the  leaves  are  shaped 
like  a  pitcher.     Xi/3- 

vegetation  is  at  the  present  time  an  important  agency  in  both 
constructively  and  destructively  modifying  the  physiographic 
features  of  the  earth. 

222.  All  forms  of  vegetation  assist  in  breaking  up  rocks  and 
dissolving  minerals  and  thus  contributing  greatly  to  the  general 
process  of  weathering  or  decay  of  rocks.  Vegetation  is  also 


IOO  PLANTS 

a  conservative  force  in  protecting  the  surface  of  the  land  from 
erosion  by  holding  the  soil  in  position  and  breaking  the  force 
of  the  rush  of  surface  waters,  which  rapidly  wears  away  naked 
or  unprotected  soils.  It  is  even  a  constructive  agency  in  many 
cases,  as,  for  example,  in  the  formation  of  deposits  of  coal,  peat, 
iron  ore  and  other  minerals,  and  in  filling  up  swamps,  ponds, 


FIG.  56. — A  solid-rock  surface  covered  with  lichens,  mosses  and  ferns. 

lakes  and  sluggish  streams  with  the  wash  from  the  land,  and 
even  in  places  building  out  the  land  into  the"  shallow  open  sea. 
223.  The  general  climatic  conditions  of  a  region  may  also 
be  modified  by  changes  in  the  vegetation  so  as  to  determine 
very  materially  the  social  and  economic  conditions  of  man.  The 
deforestation  of  a  region  has  in  more  than  one  instance  resulted 
in  the  practical  destruction  of  a  highly  developed  civilization. 


APPENDIX  TO  PART  I 

CLASSIFICATION  OF  PLANTS 

224.  BRANCH  I.  T hallo phytes. — To  this  division  of  plants 
belong  all  of  the  lowest  eleven  classes.  No  single  positive 
character  is  common  to  all.  The  plant  body  is  never  differ- 
entiated into  true  roots,  stems  and  leaves.  Classes  3-8  con- 
tain chlorophyll  and  are  called  algae.  Classes  9-1 1  are  devoid 
of  chlorophyll  and  are  called  fungi. 


FIG.  57. — A  slime  mold  creeping  over  dead  grass.     X  i. 

225.  Class  i.  Myxomycetes. — This  group  is  sometimes 
classed  with  animals  under  the  name  mycetozoa.  The  organism 
is  saprophytic  and  is  found  creeping  about  on  moist  organic 
matter,  as  on  the  vegetable  mold  under  trees.  In  the  active 
condition  a  slime-mold  plasmodium  might  be  likened  to  a 
gigantic  amoeba,  often  several  inches  in  diameter,  with  numer- 

101 


102  CLASSIFICATION   OF  PLANTS 

ous  nuclei.  It  creeps  about  by  an  amoeboid  motion  for  a  time 
but  then  comes  to  rest.  The  central  part  of  the  mass  becomes 
transformed  into  spores  while  the  superficial  parts  form  a 
peridium  or  spore  case  which  opens  when  the  spores  are  ripe 
and  permits  them  to  scatter  as  a  dry  brown  powder.  When 
the  spore  germinates  an  active  swarmspore  with  a  flagellum 
emerges.  This  greatly  resembles  a  flagellate.  After  a  time 
the  flagellum  is  lost  and  the  organism  assumes  an  amoeboid 
(Myxamceba)  condition.  By  the  fusion  of  a  number  of  these 
myxamcebae  the  plasmodium  is  formed  but  in  this  fusion  the 
nuclei  are  not  concerned.  There  is  nothing  resembling  a 
sexual  method  of  reproduction.  The  vast  number  of  spores 
produced  results  from  the  division  of  the  nuclei  of  the  plas- 
modium. A  few  slime-molds  are  parasitic  in  plants.  There 
is  no  chlorophyll  and  the  reason  for  placing  these  organisms 
under  the  plants  rests  on  the  very  unanimal-like  condition  of 
the  organism  in  the  " fruiting"  stage. 

226.  Class  2.     Schizophyta. — The  organisms   belonging  to 
this  class  are  very  simple  in  structure.     There  is  no  well-defined 
nucleus  and  the  cells  are  usually  very  small.     There  is  no 
sexual  reproduction  and  multiplication  takes  place  by  fission. 
The  cells  are  either  free  or  adhere  in  chains,  plates  or  masses 
held  together  by  the  gelatinous  cell  wall. 

227.  Order   i.     Bacteria. — The  bacteria  are  the  smallest  organisms. 
Many  forms  are  so  minute  that  even  with  the  highest  power  of  the  micro- 
scope they  appear  as  little  more  than  a  point.     There  is  a  cell  membrane 
but  no  nucleus.     Certain  granules  scattered  in  the  cytoplasm  stain  like 
chromatin  and  are  therefore  supposed  to  represent  the  nucleus.     The 
bacteria  contain  no  cholorophyll,  consequently  most  are  saprophytic  or 
parasitic.     But  some  forms  are  holophytic.     Some  forms  have  one  or 
more  cilia  and  are  motile,  others  are  motionless  and  may  be  embedded  in 
a  jelly  formed  by  the  swelling  of  the  cell  membrane.     There  is  compara- 
tively little  variety  in  form  because  of  the  simplicity  of  structure.     Never- 
theless there  are  many  species  and  these  can  be  distinguished  by  their 
physiological  characters.     The  half  dozen  type  forms  which  commonly 


BACTERIA  103 

occur  have  been  given  special  names  as  follows:  A  coccus  is  spherical 
in  form,  a  bacterium  or  bacillus  is  short  rod  shaped,  a  bacillus  slightly 
bent  is  a  vibrio,  while  one  more  strongly  curved  is  a  spirillum;  straight 
thread-like  forms  are  called  leptothrix  and  corkscrew  forms  are  spirochaete. 


FIG.  58. — Bacilli  of  various  forms.     (From  Williams.) 

228.  After  division  the  cells  may  adhere  in  chains  or  become  free.  When 
the  cell  walls  become  gelatinous  the  cells  adhere  in  a  large  mass  which  is 
known  as  a  zooglea.  Spores  are  formed  by  the  contraction  of  a  part  of 
the  protoplasm  into  a  dense  mass  which  then  surrounds  itself  with  a  cell 


*  ->' 


>V^y 

VVT/VV~ 


FIG.  59. — Spirilla  of  various  forms.     (From  Williams.) 

membrane.     Because  of  their  mode  of  formation  these  are  called  endo- 
spores.     They  are  highly  resistant. 

229.  The  physiological  differences  between  bacteria  are  very  great. 
This  is  evident  in  the  substances  which  they  excrete  and  the  effect  pro- 
duced by  these  excretions  on  the  surrounding  medium.  A  number  of 


*£ 


FIG.   60. — Staphylococci.   Streptococci.   Diplococci.  Tetrads.   Sarcinae. 
(From  Williams.) 

examples  will  be  mentioned.  Closteridium  butyricum  and  many  other 
bacteria  thrive  where  there  is  no  free  oxygen.  This  is  possible  because 
they  have  the  power  of  decomposing  substances  containing  oxygen.  On 
the  other  hand,  Bacillus  aceti,  and  others,  cause  the  combination  of  alcohol 
and  oxygen  to  form  acetic  acid  (vinegar) .  The  nitrite  bacteria  in  the  soil 
oxidize  ammonia  into  nitrites  and  the  nitrate  bacteria  continue  the  oxi- 


104  CLASSIFICATION   OF  PLANTS 

dation  into  nitrates.  These  bacteria  and  others  are  holophy  tic,  assimilating 
carbon  dioxide  like  green  plants.  Beggiatoa  alba  grows  in  water  where 
there  is  sulphuretted  hydrogen,  H2S,  formed  by  the  decomposition  of 
organic  matter.  The  bacterium  causes  the  oxidation  of  the  H2S  whereby 
the  sulphur  is  set  free  and  deposited  in  the  cell  in  small  granules.  Lepto- 
thrix  ochraea  oxidizes  iron  carbonate  into  iron  oxide  (iron  ore)  which 
is  likewise  deposited  in  the  cell. 

230.  Through  the  ferments  formed  by  many  bacteria  sugar  is  formed 
from  starch  arid  the  sugar  is  then  split  into  alcohol  and  carbon  dioxide, 
C6Hi2O6=2C2H60+2C02.  This  is  the  common  type  of  fermentation 
which  takes  place  in  the  making  of  bread,  wine,  beer  and  other  alcoholic 


FIG.  61. — Staphylococcus  aureus.     (Williams.) 

fluids.  Another  common  type  of  fermentation  is  produced  by  the  Bacillus 
vulgaris  and  other  bacteria.  The  process  here  is  ordinarily  spoken  of  as 
decay.  Nitrogenous  substances,  like  flesh,  are  decomposed  and,  among 
other  products,  sulphuretted  hydrogen  is  set  free.  It  is  this  gas  which 
produces  the  evil  odor  so  characteristic  of  this  type  of  bacterial  activity. 

23 1.  Among  the  parasitic  bacteria  are  some  which  cause  little  or  no  harm 
to  the  host.  Some  may  even  be  useful,  as  when  those  inhabiting  the 
digestive  tract  assist  in  the  process  of  digestion  (Bacillus  coli  communis). 
But  again  others  may  be  the  cause  of  the  most  malignant  and  contagious 
diseases.  Species  of  Streptococcus  and  Staphylococcus  are  generally 


ALGAE  IO5 

the  causes  of  local  eruptions,  such  as  boils,  ulcers,  gangrene,  etc.  Bacillus 
typhi  in  the  digestive  tract  causes  acute  inflammation — typhoid  fever. 
Bacillus  pneumoniae  and  the  B.  diphtheriae  on  the  mucous  epithelium  of 
the  pharynx  and  adjacent  cavities,  and  B.  tuberculosis  in  the  lungs  and 
on  other  serous  membranes  of  the  body  are  well  known.  B.  tetani  in 
the  blood  is  the  cause  of  lock-jaw.  B.  anthracis  causes  a  disease  fatal  to 
cattle  and  occasionally  to  man.  Asiatic  cholera,  leprosy  and  many  dis- 
eases of  domestic  animals,  such  as  chicken  cholera,  foot  rot,  black  leg,  etc., 
are  bacterial. 

232.  Order  2 .    Cyanophycea  (Schizophycea) . — The  Cyanophyceae  are  also 
called  blue-green  algae  because  of  the  presence  of  a  blue  pigment  (Phyco- 
cyanin)  in  addition  to  chlorophyll.     These  plants  are  found  only  in  water 
or  on  moist  surfaces.     They  multiply  by  fission  like  the  bacteria  and  the 
cells  adhere  in  threads  or  are  enclosed  in  masses  of  jelly  formed  by  the 
swollen    cell    membranes.     The    nucleus    usually  consists  of    scattered 
chromatin  granules. 

233.  Class  3.     Diatomeae. — The  Diatoms  are  a  large  group. 
They  are  also  unicellular  and  the  cells  separate   completely 
though  they  may  adhere  in  chains  or  be  attached  by  a  common 
stalk.     The  cells  are  usually  bilaterally  symmetrical  and  the 
cell  wall  consists  of  a  silicious  capsule  of  two  parts  which  fit 
into  each  other.     The  surface  of  the  capsule  is  often  very 
elaborately   ornamented.     There  is   a  single   central  nucleus 
and   one   or   more  large  lobed   chromatophores   containing  a 
brownish-yellow  pigment  in  addition  to  a  substance  similar 
to  chlorophyll.     Multiplication  takes  place  asexually  by  fission 
and  also  by  conjugation. 

234.  Class  4.     Conjugatae. — The  Conjugate  are  unicellular, 
though  the  cells  may  be  connected  in  filaments.     The  cell  has 
a  single  nucleus,  one  or  more  chlorophyll  green  chromatophores 
of  a  complicated  form  and  one  or  more  pyrenoids.     Sexual 
reproduction  through  the  union  of  two  non-motile  gametes 
(conjugation)  to  form  a  zygospore,  is  characteristic  of  the  group. 

235.  Order  i. — The  Desmidiacece  are  single  cells  which  consist  of  two 
symmetrical  halves  often  joined  by  a  narrower  portion  like  a  dumb-bell, 
the  cells  are  frequently  very  bizarre  in  form.     The  nucleus  lies  in  the 


106  CLASSIFICATION   OF  PLANTS 

narrower  middle  part  of  the  cell  and  the  chromatophore  is  symmetrically 
doubled. 

236.  Order  2. — The  Zygnemacece  are  always  filamentous  and  the  cells 
are  cylindrical.     In  conjugation  the  entire  contents  of  the  cells  is  involved. 

237.  Order  3. — The  Mesocarpacece  are   similar  to  the  foregoing  but 
only  a  part  of  the  cell  contents  is  concerned  in  conjugation. 

238.  Class  5.     Chlorophyceae. — The    Chlorophyceae    are   a 
large  group  of  fresh  water  and  marine  chlorophyll  green  algae. 
They  reproduce  asexually  by  the  formation  of  pear-shaped  zoo- 
spores  which  have  two  or  four  flagellae.     Sexual  reproduction 
usually  consists  in  the  conjugation  of  similar  zoospores  but 
there  is  often  a  differentiation  of  gametes  into  eggs  and  sperms. 

239.  Order  i. — The  Volvocales  are  motile   throughout  life.     They  are 
usually  single  and  resemble  the  green  flagellates,  but  some  forms  adhere 
by  their  gelatinous  walls  and  form  swimming  colonies.     The  cell  has  a 
single  nucleus  and  a  chromatophore. 

240.  Order  2. — The  Protoccocales  are  similar  to  the  Volvocales  but  are 
only  motile  in  the  zoospore  stage. 

241.  Order  3. — The  Ulotrichales  are  usually  simple  or  branched   fila- 
mentous forms  but  some  marine  species  form  flat  ribbons  of  two  layers  of 
cells.     The  cells  are  uninuclear  and  have  usually  one  chloroplast. 

242.  Order  4. — The  Siphonocladiales  are  also  filamentous  forms,  usually 
much  branched.     The  filaments  are  composed  of  large  multinuclear  cells 
with  one  or  more  chloroplasts. 

243.  Order  5. — The  Siphonales  consist  of  a  branching  tubular  thallus 
with  few  or  no  cross  walls  in  the  vegetative  condition.     The  protoplasmic 
substance  is  therefore  continuous,  with  numerous  nuclei  and  chloroplasts. 

244.  Class   6.     Characeae. — The  Characeae  are   fresh-water 
algae  of  rather  complicated  structure  and  with  highly  differ- 
entiated gametes  and  gametangia.     The  principal  axis  of  the 
thallus  consists  of  alternately  long  and  short  tubular  cells  form- 
ing nodes  and  internodes.     A  whorl  of  branches  occurs  at  each 
node  and  the  branches  resemble  the  main  axis  in  structure. 
Short  branches  of  a  second  order  may  also  occur  and  in  the 
axils  of  these  are  found  the  oogonia  and  antheridia.     The  first 


ALGAE  107 

consist  of  an  egg  cell  surrounded  by  a  wall  composed  of  five 
spirally  wound  branches.  The  antheridium  is  a  complicated 
spherical  structure  with  a  wall  of  eight  cells  and  containing  a 
large  number  of  spermatozoids  each  provided  with  two  flagellse. 
No  swarm  spores  are  produced. 

245.  Class  7.    Phaeophyceae. — The    Phaeophyceae    are    the 
brown  sea  weeds.     Only  a  few  small  forms  are  found  in  fresh 
water.     Among  the  marine  forms,  however,  are  the  largest  of 
all    Cryptogams.     Macrocystis  pyrifera    is    said  to  attain  a 
length  of  over  200  feet.     The  plants  are  usually  attached  to 
rocks  by  means  of  a  hold-fast  organ.     No  general  statement 
can  be  made  concerning  the  form  but  in  the  larger  species  the 
thallus  is  usually  flattened  and  often  forms  broad  sheets.     The 
cells  are  uninuclear  and  contain  a  number  of  chromatophores 
which  contain  a  brown  pigment,  phycophaein. 

246.  Order  i . — The  Phceosporecz  reproduce  asexually  by  means  of  swarm 
spores  produced  in  large  numbers  in  "unilocular"  sporangia.     Sexual 
reproduction  occurs  also   through   the  conjugation  of  motile   gametes 
developed   in   multilocular  sporangia   (gametangia)    one   gamete   being 
developed  from  each  cell  of  the  sporangium. 

247.  Order  2. — The  Cyclosporea  are  farther  advanced  sexually.     There 
is  a  marked  differentiation  of  egg  and  sperm.     In  one  family,  Dictyo- 
taceae,  asexual  aplanospores  are  also  produced  but  in  the  Fucaceae  there  is 
no  asexual  reproduction. 

248.  Class  8.    Rhodophyceae. — The  Rhodophyceae  or  Flor- 
ideae  are  the  red  sea  weeds.     A  few  forms  occur  in  fresh  water. 
The  red  sea  weeds  are  small  as  compared  with  the  brown.     The 
form  of  the  thallus  is  most  often  a  bushy  mass  of  branching 
delicate  filaments  or  of  thin  sheets.     Some  species  are  encrusted 
with  calcium  carbonate.     The  color  is  due  to  a  red  pigment, 
phycoerythrin,  found  in  the  chromatophores  in  addition  to  the 
green.     Asexual  reproduction  takes  place  through  non-motile 
tetraspores,  which  are  produced  in  groups  of  four  on  the  surface 
of  the  thallus.     The  sexual  reproduction  is  peculiar.     The  male 


108  CLASSIFICATION   OF  PLANTS 

cells  are  minute  non-motile  "spermatia,"  cut  off  from  the  tips 
of  certain  branches.  The  female  branch,  carpogonium,  is  ter- 
minated by  a  slender  filament,  the  trichogyne.  When  a  sper- 
matium  comes  in  contact  with  this  trichogyne  a  fusion  takes 
place  which  effects  a  fertilization.  The  result  is  that  by  a  more 
or  less  indirect  process  spores,  carpospores,  are  developed 
farther  down  on  the  branch. 

249.  Class  9.  Phycomycetes. — The  Phycomycetes  are  fungi, 
that  is,  they  contain  no  chlorophyll  and  are  therefore  sapro- 
phytic  or  parasitic  in  habit.  But  in  many  other  respects  they 
resemble  algae,  especially  the  Siphonales.  The  thallus  is  tubu- 
lar with  few  or  no  cross  walls  dividing  it  into  cells,  and  the  proto- 
plast contains  numerous  nuclei.  Spores  are  formed  asexually 
by  the  division  of  the  protoplasmic  contents  of  a  sporangium. 
These  spores  are  motile  in  aquatic  forms  but  those  of  terrestrial 
forms  are  simple  rounded  cells  which  are  scattered  like  dust. 
In  some  genera  a  kind  of  spore,  conidium,  is  formed  by  the 
cutting  off  of  a  cell  from  the  tip  of  a  filament  (hypha) . 

2  50.  Order  i . — The  Oomycetes  reproduce  sexually  by  means  of  sperm  and 
egg  cells.  In  some  cases  the  sperm  cells  are  motile  spermatozoids  which 
are  set  free  from  an  antheridium,  make  their  way  to  the  oogonium  and 
fuse  with  the  egg  cell  producing  an  oospore.  In  most  cases,  however, 
the  antheridium  forms  a  tube  which  grows  toward  and  into  the  oogonium. 
In  this  case  the  sperm  cells  are  not  provided  with  flagellae.  They  pass 
through  the  tube  of  the  antheridium  directly  into  the  oogonium  and  there 
reach  the  egg  cell.  The  asexual  spores  are  swarm  spores.  The  oomycetes 
grow  as  saprophytes  in  fresh  water  or  as  parasites  in  plants  and,  occa- 
sionally, animals. 

251.  Order  2. — The  Zygomycetes  reproduce  sexually  by  the  fusion  of  the 
contents  of  two  similar  gametangia.     The  resulting  body  is  called  a  zygo- 
spore.     The  asexual  spores  are  produced  either  in  sporangia  or  as  conidia. 
The  Zygomycetes  are  terrestrial  and  grow  as  saprophytes  on  vegetable  or 
animal  matter  or  as  parasites  in  insects.     The  black  mold  on  bread,  etc., 
is  a  familiar  example. 

252.  Class  10.    Basidiomycetes. — The  Basidiomycetes  are 
distinguished  by  the  club-shaped  basidium  upon  which   four 


FUNGI 

spores  are  produced  by  budding.  The  basidium  is  a  terminal 
hypha  and  is  either  unicellular  and  bears  the  four  spores  at  its 
free  end  or  else  it  is  divided  into  four  cells  each  one  of  which 
bears  one  spore. 

Rudimentary  sexual  organs  are  found  but  there  is  usually  no 
sexual  process  connected  with  the  formation  of  spores.  The 
basidia  are  usually  grouped  and  borne  on  the  surface,  or  within 
special  fruiting  bodies.  Besides  the  basidiospores  there  may  be 
also  one  or  more  other  kinds  of  spores  formed  by  the  same 
fungus. 

253.  Order   i. — Hemibasidiales   are   parasitic   on   plants.     The   black 
corn-smut  is  a  familiar  example.     A  short  hypha  which  is  supposed  to 
represent  the  basidium  is  developed  directly  from  the  thick-walled  brand 
spores.     The  mycelium  developed  from  the  basidiospores  ramifies  through 
the  tissues  of  the  host  and  ultimately  produces  large  masses  of  brand  spores. 

254.  Order  2. — Protobasidiomycetes  have  a  basidium  divided  into  four 
cells  each  of  which  bears  a  spore.     The  "rusts'.'  are  the  most  important 
members  of  this  group.     See  p.  364  for  the  life  history  of  the  wheat  rust. 
There  are  also  some  saprophytic  forms  which  have  a  gelatinous  thallus. 

255.  Order  3. — The  Autobasidiomycetes  have  the  basidium  undivided 
but  bearing  four  sterigmata  with  one  basidiospore  on  each.     The  group  is 
a  very  large  one.     In  most  cases  the  basidia  are  arranged  in  a  well-defined 
layer  called  the  hymenium  and  this  is  borne  on  a  fruiting  body  of  very 
definite  form.     The  hymenium  may  form  a  single  flat  surface,  or  it  may 
be  variously  folded  into  numerous  tooth-like  of  finger-like  projections  or 
parallel  plates.     In  other  groups  again  it  lines  the  walls  of  slender  tubes 
or  of  numerous  closed  chambers.     To  this  order  belong  the  mushrooms  and 
puffballs. 

256.  Class  1 1 .     Ascomycetes. — The  Ascomycetes  are  charac- 
terized by  the  sack-like  sporangium,  the  ascus.     This  is  formed 
from  the  terminal  cell  of  a  hypha,  the  two  nuclei  of  which  fuse 
and  then  divide,  usually  three  times  so  that  eight  spores  are 
formed.     Not  all  of  the  protoplasm  of  the  ascus  is  used  in  the 
formation  of  the  spores.     The  asci  are  usually  clustered  in 
characteristic  fruiting  bodies.     Sexual  reproduction  by  oogonia 
has  been  observed  in  a  few  cases. 


110  CLASSIFICATION   OF  PLANTS 

257.  Order  i. — The  Peris poreacea  comprise  the  mildews  and  the  blue 
molds.     In  this  group  the  asci  are  completely  enclosed  in  a  minute  spher- 
ical fruiting  body,  the  perithecium.     The  mildews  are  parasitic  on  the 
leaves  of  higher  plants  as  exemplified  by  the  common  grape  and  lilac  mil- 
dews.    The  blue  molds  are  saprophytic  on  decaying  fruits,  preserves, 
bread,  leather,  etc.,  and  are  usually  readily  distinguished  from  the  black 
molds  (Phycomycetes)  by  the  color  and  by  the  conidia. 

258.  Order  2. — In  the  Discomycetes  the  asci  are  borne  in  concave  disk- 
shaped  fruiting  surfaces  (apothecia).     The  species  of  this  order  are  very 
common  and  are  usually  saprophytic.     They  are  most  frequently  found 
on  decaying  wood.     The  edible  morel  (Morchella)  belongs  to  this  order. 

259.  Order  3. — In  the  Pyrenomycetes  the  perithecium  is  flask  shaped 
with  a  pore  through  which  the  spores  escape.     In  the  mature  condition 
the  fruiting  bodies  are  usually  black.     The  Pyrenomycetes  are  in  part 
parasitic;  some  on  other  plants  as  the  black  knot  of  plumb  trees  or  the 
ergot  of  rye  and  some  in  the  bodies  of  insect  larvae.     Many  forms  are 
saprophytic  on  bark,  decaying  wood,  etc. 

260.  Order  4. — The  Tuber acea  are  saprophytic  underground  in  forest 
humus.     The   perithecia   are   large  spherical  bodies   without    opening. 
Some  forms  are  edible. 

261.  Order  5. — The  Exoasci  are  parasitic  on  trees  and  stimulate  the 
tissues  of  the  host  to  abnormal  growth  thus  forming  in  certain  cases 
"witches'  brooms." 

262.  Order  6. — The  Saccharomycetes  (yeasts)  are  microscopic  unicellular, 
saprophytic  fungi.     They  are  important  as  the  chief  agents  in  the  various 
fermentation  processes  connected  with  the  making  of  bread,  wine,  beer 
and   other   alcoholic   liquors.     Reproduction    takes   place   by   budding 
(conidia)  and  under  favorable  conditions  spores  are  formed  endogenously 
within  a  cell  (ascus). 

263.  Lichens. — The  lichens  are  one  of  the  most  common  and  most  widely 
distributed  types  of  vegetation.     They  may  be  found  on  almost  any  kind 
of  stable  surface,  on  rocks,  tree  trunks,  or  on  the  surface  of  the  earth. 
They  are  often  confused  with  mosses  but  are  readily  distinguishable. 
The  color  is  usually  gray  or  brown  but   never  chlorophyl  green.     The 
fruiting  surfaces  are  frequently  brilliantly  colored.     The  form  of  the  plant 
is  thalloid,  never  differentiated  into  true  stem  and  leaf.     Serious  objection 
may  be  made  to  ranking  them  as  a  class  because  a  lichen  is  in  reality  only 
a  symbiotic  combination  of  an  alga  and  a  fungus,  either  of  which  may  be 
grown  independently  of  the  other.     The  algae  concerned  would  by  them- 
selves be  classed  as  Cyanophyceae  or  as  Chlorophyceae  of  the  simpler  forms. 


ARCHE  GONIATE  S  III 

The  fungi  are  usually  Ascomycetes  but  a  few  Basidiomycetes  also  occur 
as  lichens.  The  fungous  filaments  wind  about  the  algal  cells  and  usually 
form  a  firm  superficial  protective  tissue.  The  specific  characteristics  of 
a  lichen  depend  upon  both  the  alga  and  the  fungus  components  but  the 
fruiting  surface  of  the  lichen  is  purely  fungal.  (See  page  362.) 

264.  BRANCH  II.     Bryophyta. — The  Bryophyta  include  the 
liverworts  and  mosses.     They  are  distinguished  from  the  Thal- 
lophytes  by  the  structure  of  the  gametangia.     The  sperm  cells 
are  provided  with  two  long  flagellae  and  they  are  formed  in  large 
numbers  within  an  oval  capsule,  the  antheridium,  whose  walls 
are  composed  of  a  single  layer  of  cells.     The  single  egg  cell  is 
contained  in  the  lower  part  of  a  flask-shaped  gametangium, 
called  archegonium.     This  is  also  formed  by  a  single  layer  of 
cells.     The  upper  portion,  or  neck,  of  the  archegonium  contains 
an  axial  row  of  cells   (canal  cells)   which    disintegrate    and 
form  a  slime  through  which  the  spermatozoids  make  their  way 
to  the  egg.     A  " ventral  canal  cell"  cut  off  from  the  egg  cell 
at  the  lower  end  of  the  canal  cells  also  distintegrates  with  the 
canal  cells. 

265.  The   fertilized   egg  immediately  begins   development. 
There  is  ultimately  formed  an  organism  (sporophyte)  which 
produces    spores    asexually.     From    these    spores    there    then 
develops  an  organism  (gametophyte)  which  bears  the  gametan- 
gia.    In  this  way  a  regular  alternation  of  sexual  and  asexual 
generations  occurs. 

266.  The  Bryophyta  are  called  Archegoniates  from  the  very 
characteristic  female  gametangium.     They  are  always  holo- 
phytic  and  are  chlorophyll  green. 

267.  Class  i.    Hepaticae. — The  liverworts  are  usually  thalloid 
or  if  there  is  a  differentiation  into  stem  and  leaves  the  latter 
are  arranged  dorso-ventrally.     The  thallus  branches  dichoto- 
mously  and  is  attached  to  the  substratum  by  rhizoids,  i.  e.,  tubu- 
lar, hair-like  cells  which  serve  as  organs  for  the  absorption  of 
moisture.     The  spore  capsules  usually  contain  sterile  elongated 


112  CLASSIFICATION   OF  PLANTS 

cells   (elaters)   which  serve  to  scatter  the  spores  by  hygro- 
scopic movements. 

268.  Order  i. — The  Ricciacea  are  aquatic  or  semi-aquatic  thalloid  forms. 
The  sporophyte  remains  completely  enclosed  within  the  archegonium  wall 
and  embedded  in  the  gametophyte  thallus.     It  consists  merely  of  a  spher- 
ical capsule  filled  with  spores. 

269.  Order  2. — The   Marchantiacea   are  larger  and  more  complex  in 
structure.     The  thallus  is  perforated  by  pores  on  the  upper  side,  which 
open  into  air  chambers.     Surrounding  and  projecting  into  these  chambers 
are  the  green  assimilatory   cells.     The   deeper   lying   cells  are  larger, 
contain   little   or  no  chlorophyll  and  serve  for  water  storage  and  con- 
duction.    The    antheridia    and    archegonia   are   borne   on    stalked   re- 
ceptacles.    The  sporophyte  is  a  stalked  capsule  which  remains  attached 
to  the  receptacle  but  projects  beyond  the  old  archegonium  wall. 

270.  Order  3. — The  Anthocerotaceae  are  a  smaller  group.     The  game- 
tophyte is  irregular  thalloid.     Its  cells  contain  each  a  single  chloroplast. 
The  archegonia  are  embedded  in  the  thallus.     The  sporophyte  projects 
beyond  the  thallus  because  of  its  greatly  elongated  form,  but  is  not  stalked. 
The  capsule  splits  longitudinally  and  there  is  a  slender  axial  "columella" 
of  sterile  tissue. 

271.  Order  4. — The  Jungermanniacece  are  usually  differentiated  into 
a  stem  and  dorso- vent  rally  arranged  leaves  one  cell  layer  thick.     The 
capsule  is  long  stalked  and  usually  opens  longitudinally  by  four  valves. 
There  is  no  columella. 

272.  Order  5. — The  Calobryacece  are  represented  only  by  two  exotic 
genera. 

273.  Class  2.     Musci. — The  moss  plant  is  differentiated 
into  stem  and  leaves.     There  are  no  true  roots  but  at  the  base 
of  the  stem  are  found  branching  rhizoids  by  which  water  is 
absorbed.     The  stem  sometimes  contains  an  axial  strand  of 
elongated  cells  which  serve  for  the  conduction  of  fluids,  but 
there  are  no  true  nbro-vascular  bundles.     The  leaves  are  usually 
one  cell  layer  thick  and  are  arranged  spirally  on  the  stem. 

274.  The  archegonia  and  antheridia  are  borne  at  the  apex 
of  the  stem  or  on  lateral  branches.     The  sporophyte  is  stalked 
and  remains  attached  to  the  gametophyte  by  a  prolongation 


ARCHE  GONI  ATE  S  113 

of  the  stalk  known  as  the  foot.     The  capsule  contains  a  central 
axis  of  sterile  tissue,  the  columella. 

275.  When  the  spores  germinate  a  branching  green  thread 
(protonema)  like  an  alga  is  developed.     From  this  the  moss 
plants  are  formed  by  budding.     By  this  protonema  the  mosses 
may  be  distinguished  from  the  liverworts.     The  mosses  are 
always  holophytic  and  chlorophyll  green. 

276.  Order  i. — The  Sphagnacece  are  the  swamp  mosses.     They  grow 
continually  upward  while  dying  away  at  the  base.     The  capsule  is  short 
stalked  and  the  foot  is  broad.     A  pseudopodium  is  formed  by  the  elonga- 
tion of  the  branch  below  the  foot.     The  archegonium  wall  breaks  and  its 
fragments  remain  at  the  base  of  the  capsule.     The  capsule  opens  by  a  lid. 
The  columella  is  hemispherical.     There  is  only  one  genus,  Sphagnum, 
with  many  species. 

277.  Order  2. — The  Andreacea  are  usually  found  in  small  clusters  on 
rocks.     The  capsule  is  short  stalked  with  a  broad  foot  and  is  elevated  by 
a  pseudopodium.     The  archegonium  wall  breaks  around  the  base  and 
remains  on  the  capsule  like  a  cap  (calyptra).     The  capsule  opens  by  four 
longitudinal  slits.     There  is  only  one  genus,  Andrea. 

278.  Order  3. — The  Phascacece  are  small,  simple  mosses  with  persistent 
protonema,  and  a  short  stalked  capsule  which  does  not  open.     The  spores 
are  set  free  only  by  the  disintegration  of  the  capsule. 

279.  Order  4. — The  Bryina  comprise  most  of  the  common  mosses.     In 
these  the  capsule  is  long  stalked  (seta)  with  a  foot.     The  archegonium 
forms  a  calyptra.     The  capsule  opens  by  a  lid  (operculum).     On  removal 
of  the  operculum  the  edge  of  the  opening  of  the  capsule  is  seen  to  be  pro- 
vided with  a  fringe,  the  peristome,  which  by  hygroscopic  movements 
assists  in  the  scattering  of  the  spores. 

280.  BRANCH    III.    Pteridophyta. — The   ferns   are  differen- 
tiated  into  true  root,  stem  and  leaf.     Conducting  and  sup- 
porting tissues  in  the  form  of  fibre-vascular  bundles  occur,  as  in 
the  higher  plants.     The  ferns  are  like  the  mosses  in  regard  to  the 
structure  of  the  gametangia  and  are  hence  archegoniata.     Like 
the  mosses  they  also  have  a  distinct  alternation  of  generations. 
The  gametophyte,  however,  is  reduced  to  an  inconspicuous 
thalloid  structure  (prothallium)   or  still  farther  to  a  minute 

8 


114  CLASSIFICATION    OF   PLANTS 

cluster  of  colorless  cells.     The  sporophyte,  on  the  other  hand 
is  much  better  developed  and  constitutes  the  leafy  plant. 

281.  Class  i.    Filicinae. — The  Filicinae  are  the  true  ferns, 
a  large  group  of  plants  of  moderate  size.     A  few  tropical  "  tree 
ferns"  attain  the  size  of  a  small  tree  but  the  more  familiar 
forms  have  only  an  underground  stem  (root-stock)  from  which 
the  leaves  (fronds)  rise  on  long  petioles  (stipes)  to  a  height  of 
from  i  to  5  feet.     All  ferns  are  holophytic.     Many  species, 
especially  tropical  ones,  are  epiphytic. 

282.  The  gametophyte  of  the  fern  is  a  small  green  thalloid 
structure   (prothallium)   which  lies  flat  on  the   ground.     (Or 
colorless,  saprophytic  and  underground,  Order  i.)     Embedded 
in  its  tissues  are  the  antheridia  and  archegonia.     The  anthero- 
zoids  are  spiral  bodies  with  a  tuft  of  cilia  at  one  end.     The 
fertilized  egg  cell  divides  into  four  segments  from  which  are 
developed  root,  stem,  leaf  and  foot  respectively.     The  foot  is 
an  organ  by  which  the  developing  plant  retains  connection 
with   the  prothallus  for  some   time.     The  prothallus  finally 
disintegrates    and    the    plantlet   becomes    independent.     The 
plant  with  the  root,  stem  and  leaves  is  the  sporophyte.     The 
spores  are  developed  in  sporangia  on  the  under  surface  of  the 
leaves.     Sometimes  the  spore-bearing  portion  of  a  leaf  is  espe- 
cially modified,  or  again  the  spores  are  only  borne  on  certain 
leaves  which  then  are  completely  modified  (sporophylls) . 

283.  Order  i. — The  Ophioglossacea.  or  adder-tongue  ferns,  are  a  small 
group  of  slow  growing  and  rather  inconspicuous  plants.     The  gametophyte 
is  a  small,  saprophytic,  underground  thallus.     The  leaf  is  partly  differ- 
entiated into  sporophyll. 

284.  Order  2. — The  Marattiacece  are  tropical  ferns  of  large  size.     The 
prothallus  is  a  green  thallus  resembling  a  liverwort.     The  sporangia  are 
grouped  in  sori  on  the  under  surface  of  the  foliage  leaves. 

285.  Order  3. — The  Filices  are  the  order  to  which  most  of  our  common 
ferns  belong.     Most  of  the  tropical  tree  ferns  also  belong  to  this  order. 
The  order  comprises  many  genera  and  species.     Many  are  epiphytic. 
The  gametophyte  is  usually  a  small,  green,  liverwort-like  prothallus  which 


ARCHE  GONIATE  S  115 

bears  antheridia  and  archegonia  on  its  under  surface  embedded  in  its 
tissue.  The  sporophyte  usually  bears  the  spores  on  the  under  surface 
of  undifferentiated  leaves.  The  sporangia  are  stalked  and  grouped  in 
clusters  (sori) .  The  sori  are  often  covered  by  a  scale  (indusium) . 

286.  Order  4. — The  Hydropteridece  or  water-ferns,  are  a  small  group  of 
plants  which  bear  little  resemblance  to  common  ferns.     Some  grow  in 
the  mud,  partly  submerged,  others  float  on  the  surface  of  the  water. 
They  are  of  special  biological  interest  because  the  sporophyte  bears  two 
kinds  of  spores,  small   "  microspores "  and  large  "megaspores."     The 
microspores  develop  a  very  simple  prothallus  consisting  of  only  a  few 
colorless  cells,  and  a  few  antheridia.     The  megaspores  develop  a  slightly 
larger  prothallus  which,  however,  only  projects  slightly  beyond  the  broken 
sporangium  wall.     (A  megasporangium  produces  only  one  megaspore.) 
A  few  archegonia  are  formed  in  the  prothallus  but  only  one  egg  cell  devel- 
ops.    The  microspores  therefore  develop  male  gametophytes  and  the  mega- 
spores female. 

287.  Class  2.    Equisetinae. — This  class  contains  only  one 
genus,  Equisetum,  the  common  scouring  rush  or  "horse-tail." 
These  plants  have  an  underground  stem  from  which  the  erect 
fruiting  and  vegetative  stems  rise  each  season.     The  stem  is 
fluted  and  jointed  and  green  since  there  are  no  foliage  leaves. 
The  scale  leaves  sheathe  the  stem  at  the  nodes.     The  fruiting 
stems  are  usually  simple  while  the  vegetative  stems  bear  whorls 
of  branches  at  the  nodes.     The  epidermis  is  encrusted  with 
silica  which  gives  the  stem  a  harsh  feel  and  lends  the  name 
scouring  rush.     The  fruiting  stems   bear   a   conical  spike   of 
umbrella-shaped  sporophylls.     The  spores  are  all  of  one  kind 
and  are  each  provided  with  four  ribbon-like  hygroscopic  appen- 
dages (elaters)  by  which  the  spores  are  scattered.     The  spores 
give  rise  to  a  branching  prothallus  which  is  usually  unisexual. 

288.  Class    3.     Lycopodinae. — The  Lycopodinae   are  small 
plants  with  some  affinities  to  the  ferns  but  of  very  different 
appearance.     The  sporophylls  each  bear  a  single  sporangium. 

289.  Order  i. — The  Lycopodiacece  are  the  lycopodiums,  "trailing  cedar" 
or  "ground  pine."     The  stem  is  usually  trailing,  with  short  ascending 
branches.     Branching  of  stem  and  roots  is  dichotomous.     The  leaves  are 


Il6  CLASSIFICATION   OF   PLANTS 

broad  awl  shaped  and  thickly  set.  The  sporophylls  are  usually  borne 
in  a  spike  with  the  sporangia  on  the  upper  side  of  the  scale-like  leaves. 
The  spores  are  all  of  one  kind.  The  gametophyte  is  bisexual.  It  is  a 
club-shaped  saprophytic  organism  in  some  cases,  in  others  it  forms  a  flat 
green  thallus.  The  spermatozoids  are  provided  with  two  flagellae.  The 
embyro  develops  a  suspensor  as  in  Selaginella. 

290.  Order  2. — Selaginellacea  also  all  belong  to  one  genus,  Selaginella. 
Most  of  the  species  are  tropical  but  a  few  delicate  moss-like  forms  are 
found  in  our  forests.     The  plant  resembles  lycopodium  somewhat  but 
the  arrangement  of  the  leaves  is  usually  dorso-ventral.     There  are  fre- 
quently two  dorsal  rows  of  very  small  leaves  and  two  ventral  rows  of  larger 
ones.     The  sporangia  are  borne  in  the  axils  of  leaves  near  the  tip  of  a 
branch.     There  are  two  kinds  of  spores  found  in  the  same  spike.     Some 
sporangia  contain  four  megaspores,  others  numerous  microspores.     The 
microspores  develop  into  a  prothallium  of  one  cell  and  an  antheridium 
of  eight  cells  within  which  a  number  of  spermatozoids,  each  with  two 
flagellae,  are  formed.     The  megaspore  develops  a  small  colorless  prothal- 
lium in  which  a  few  archegonia  are  formed.     Only  one  or  two  of  the  arche- 
gonia  are  fertilized.     The  embryo  develops  an  appendage,  the  suspensor, 
which  consists  of  a  row  of  cells,  by  which  the  embryo  is  pushed  down  into 
the  nourishing  prothallium. 

291.  Order  3. — The  Isoetacece  consist  of  the  single  genus,  Isoetes.     The 
plants  are  small,  with  long  needle-shaped  leaves  arranged  in  a  rosette 
around  a  short  erect  stem.     The  plants  are  found  submerged  in  water 
or  in  wet  soil.    The  sporangia  are  borne  on  the  inner  surface  of  the  leaves, 
at  the  base.     The  outer  leaves  bear  megasporangia,  the  inner  ones  micro- 
sporangia.     The  spermatozoids  are  spiral  and  have  a  tuft  of  cilia  like 
those  of  the  ferns.     The  embryo  has  no  suspensor. 

292.  BRANCH  IV.  Spermatophytes. — The  three  branches  of 
the  vegetable  kingdom  already  described  are  together  called 
Cryptogams  and  in  distinction  to  them  all  the  higher  forms  are 
called  Phanerogams.  The  latter  are  in  general  more  highly 
developed,  but  the  distinguishing  character  is  the  seed,  like 
which  nothing  is  found  among  the  Cryptogams.  In  the  Phan- 
erogams the  female  gamete  is  developed  within  the  megaspore 
wall  and  the  egg  cell  is  fertilized  and  develops  an  embryo  while 
the  megaspore  is  still  within  the  sporangium  and  attached  to 
the  parent  sporophyte.  After  the  embryo  is  well  formed 


SPERMATOPHYTES  117 

.development  comes  to  a  standstill  and  the  sporangium  with  the 
enclosed  embryo  is  set  free.  This  structure  is  the  seed.  The 
seed-bearing  plants  are  called  Spermatophytes  and  form  a 
branch  coordinate  with  Thallophytes,  Bryophytes  and  Pteri- 
dophytes.  In  the  Spermatophytes  alternation  of  generation 
occurs  as  in  the  Archegoniates  but  the  gametophyte  is  re- 
duced even  further  than  in  the  higher  Pteridophytes. 

293.  Class  i.  Gymnospermae. — In  the  Gymnosperms  the 
sporangia  are  borne  on  the  surface  of  modified  leaves  (sporo- 
phylls).  The  microsporophylls  are  arranged  spirally  on  a 
short  branch.  The  megasporophylls  are  usually  similarly 
arranged.  The  microspore  (pollen  grain)  develops  a  rudimen- 
tary prothallus  of  from  one  to  three  cells  and  the  sperm  cells 
reach  the  megaspore  through  a  tube  developed  by  the  game- 
tophyte. The  megaspore  (embryo  sac)  develops  a  prothallus 
of  many  cells  and  several  archegonia.  The  latter  consists  of 
a  large  egg  cell  and  a  few  small  neck  cells. 

294.  Order  i. — The  Cycadince  are  tropical  palm-like  plants,  with  an 
tmbranched  trunk  and  a  rosette  of  pinnate  leaves.     The  sperm  cells  bear  a 
spiral  band  of  cilia,     The  embryo  consists  of  a  suspensor,  two  cotyledons, 
a  plumule  and  a  hypocotyl. 

295.  Order  2. — The  Ginkgoina  contain  only  the  Japanese  genus  Ginkgo, 
a  deciduous  tree  with  small  fan-shaped  palmately  veined  leaves.     The 
sperm  cells  are  ciliated.     The  embryo  forms  no  suspensor. 

296.  Order  3. — The  Coniferce  comprise  the  "evergreens"  and  a  few 
deciduous  trees.     The  pines,  cedars,  spruces,  hemlocks,  cypresses  and 
junipers  are  familiar  examples.     In  the  structure  of  the  stem  they  differ 
from  most  Spermatophytes  in  the  absence  of  tracheae.     The  tracheids  are 
highly  developed  and  take  the  place  of  tracheae.     The  sperm  cells  are  not 
ciliated.     The  embryo  forms  a  suspensor. 

297.  Order  4. — The  Gnetince  contain  only  three  genera  of  exotic  plants. 
They  constitute  in  many  respects  a  connecting  link  between  the  Gymno- 
sperms and  Angiosperms. 

298.  Class    2.    Angiospermse. — The   Angiosperms  include 
all  the  true  flowering  plants.     The  most  distinctive  character 


Il8  CLASSIFICATION   OF   PLANTS 

of  the  group  is  the  pistil,  a  megasporophyll  so  formed  as  to 
entirely  enclose  the  megasporangia.  The  flower  consists  of 
several  circles  of  sporophylls  surrounded  by  several  circles  of 
specially  modified  floral  leaves.  All  these  leaves  are  set  close 
together  on  an  extremely  short  axis.  The  typical  flower  bears 
at  its  apex  or  centre  a  circle  of  megasporophylls,  around  this 
two  circles  of  microsporophylls  and  around  these  again  two 
circles  of  floral  leaves.  Both  circles  of  floral  leaves  may  be 
colored  or  only  the  inner  one. 

299.  The  male  gametophyte  is  represented  only  by  a  single 
pollen  tube  nucleus  and  a  sperm  mother  nucleus  which  divides 
into  two  sperm  nuclei.     No  division  into  cells  occurs.     The 
female  gametophyte  is  represented  by  the  endosperm  and  the 
archegonium  by  the  egg  apparatus  (two  synergids  and  the  egg 
cell). 

300.  The  first  seven  orders  of  Angiosperms  are  Monocoty- 
ledonous    (see    page    71).     The  Dicotyledons  contain  thirty- 
three  orders. 

301.  For  a  detailed  description  of  flowering  plants  the  student 
should  consult  a  manual  of  botany. 


PART  II.-ANIMALS 

LABORATORY  EXERCISES 

I.  Organization  of  Animals 

90.  Protozoa: 

(a)  Observe  an  actively  moving  amoeba  for  some  time  and  sketch  its 
outline  five  times  to  show  the  change  of  form.     Trace  in  these 
outlines  the  changes  through  which  each  pseudopodium  passes. 
Note  the  ingested  food  particles  and,  if  possible,  observe  the 
process  of  ingestion.     Note  the  contractile  vacuole. 

(b)  In  a  stained  preparation  note  the  structure  of  the  protoplasm, 
the  nucleus,  the  contractile  vacuole  and  the  food  vacuoles. 

91.  (a)  Study  the  movement  of  a  ciliate  protozoan  (Paramecium).     How 

many  kinds  of  movement  does  it  perform. 

(b)  Study  a  living  individual  under  higher  magnification.     Note  the 
cilia,  the  buccal  groove  leading  to  the  mouth,  the  food  vacuoles 
and  the  contractile  vacuoles. 

(c)  In  a  stained  preparation  note  the  macronucleus  and  the  micro- 
nucleus. 

92.  Ccelenterata: 

(a)  Observe  a  living  hydra  in  the  aquarium,  first  with  the  unaided 
eye  then  with  the  lens. 

(b)  A  hydra  (living  or  a  fixed  preparation)  which  shows  reproduction 
by  budding. 

(c)  Preparations  of  hydra  to  show  the  gonads. 

(d)  A  cross  section  of  the  body  to  show  ectoderm  and  endoderm. 
Note  the  muscle  fibrils  which  show  as  dots  between  ectoderm 
and  entoderm.     Also  the  central  gastro-vascular  cavity. 

93.  (a)  A   hydroid   colony    (Obelia)    (Pennaria).     Sketch   the   colony. 

Compare  a  polyp  with  hydra.     Is  there  evidence  of  budding? 
(b)  A  hydrozoan  medusa  (Obelia).     In  a  stained  preparation  note 

the  manubrium  and  mouth,  the  radial  canals,  the  gonads,  the 

tentacles  and  the  velum. 
(b7)  The  medusa  of  Gonionemus  is  larger  than  that  of  Obelia  and  may 

be  studied  with  the  lens. 


120  ANIMALS 

94.  Annelida: 

(a)  If  living  Nereis  is  at  hand  study  its  movements. 

(b)  In  a  dorsal  view  of  Nereis  note  general  form  of  body;  head;  sen- 
sory,  locomotor,    and   respiratory   appendages;    segmentation; 
symmetry. 

(c)  In  a  small  living  specimen  the  dorsal  blood  vessel  may  be  seen. 
Note  its  rhythmical  contractions.     Note  the  direction  of  the 
flow. 

(d)  Study  the  head  with  a  lens.     Note  the  proboscis,  tentacles,  palps, 
cirri  and  eyes. 

(e)  Study  a  segment  cut  from  the  middle  of  the  body.     Note  the 
four  large  muscle  masses,  the  intestine,  the  body  cavity,  the 
dorsal  and  ventral  blood  vessels,  the  ventral  nerve  cord  and  the 
parapodia. 

(f)  On  a  parapodium  note  its  two  divisions — dorsal  and  ventral 
rami — each  bearing  a  cirrus,  a  ligula,  setigerous  lobes,  setae  and 
an  acicuhim. 

(g)  In  a  portion  of  the  body  from  which  the  dorsal  wall  has  been 
removed,  note  the  intestine,  the  body  cavity  and  the  mesenteries. 

(h)  In  a  microscopic  preparation  study  the  ova. 

95.  Compare  the  earthworm  (Lumbricus)  with  Nereis. 

96.  Some  of  the  smaller  fresh-water  annelids  may  be  studied  living,  as 
transparent  objects,  under  the  microscope. 

97.  Arthropoda: 

(a)  If  living  crawfish  (Cambarus)  or  lobsters  (Homarus)  are  available 
study  the  movements.     Note  how  the  tail  fin  is  used  in  loco- 
motion, also  the  legs.     By  adding  a  little  carmine  to  the  water 
can  you  detect  any  respiratory  currents?     What  causes  them? 
Feed  with  an  earthworm  and  note  activity  of  sense  organs  and 
the  method  of  ingestion  of  food. 

(b)  In  a  dorsal  view  note  type  of  symmetry  and  character  of  seg- 
mentation of  the  body. 

(c)  In  a  lateral  view  note  in  the  cephalo thorax:  rostrum,  head,  nuchal 
groove,  thorax.    In  the  abdomen:  segments  (No.  ?),  telson.    The 
appendages  of  the  cephalothorax  are:  i  antennules,  2  antennae, 
(eyes),  3  mandibles,  4  first  maxillae,  5  second  maxillae,  6  first 
maxillipeds,   7  second  maxillipeds,  8  third  maxillipeds.     (For 
appendages  3  to  8  see  d).     9  Chelae,  10-13  ambulatory  appen- 
dages.    Are  these  all  alike?     The  appendages  of  the  abdomen 
are:  14-18  pleopods,  19  uropods.     Are  the  pleopods  all  alike? 


LABORATORY   EXERCISES  121 

Compare  pleopods  of  male  and  female.  Study  with  a  lens  a 
pleopod  of  segment  1 6  or  17;  there  is  a  basal  protopod,  a  lateral 
exopod  and  a  medial  endopod. 

(d)  Study  the  region  of  the  mouth  in  a  ventral  view,  to  show  espe- 
cially the  appendages  3-8  (see  c  above).     Begin  with  appendage 
8  and  work  forward. 

(e)  In  a  ventral  view  of  the  entire  animal  note  the  openings  of  the 
green  glands  on  the  basal  segments  of  the  antennae,  the  openings 
of  the  gonoducts  at  the  base  of  the  eleventh  (female)  or  thirteenth 
(male)  appendages,  and  the  anal  opening.     Draw  the  appendages 
9-13  in  detail,  showing  all  the  joints. 

(f)  Note  the  sensory  hairs.     Where  are  they  found?     In  the  eyes 
note  the  eye  stalk  and  the  retina.     Study  a  preparation  of  the 
retina  with  the  microscope.     With  the  point  of  a  needle  search 
the  dorsal  surface  of  the  basal  segment  of  the  antennule  for  the 
opening  into  the  statocyst. 

(g)  In  a  specimen  in  which  the  gill  chamber  has  been  laid  open 
note  character  of  the  gills,  their  number,  position  and  mode  of 
attachment  to  the  body.     With  a  lens  study  a  single  gill  under 
water  in  a  watch  glass. 

(h)  In  a  specimen  that  has  been  sectioned  longitudinally  near  the 
median  plane  note:  (i)  the  digestive  tract  with  oesophagus, 
cardiac  and  pyloric  portions  of  the  stomach,  the  liver  and  the 
intestine;  (2)  the  heart  in  the  pericardial  chamber  under  the 
posterior  edge  of  the  thoracic  shield  and  the  arteries  (ophthal- 
mic, sternal  and  abdominal),  (3)  the  gonads  lying  below  the 
heart,  (4)  the  green  gland  above  the  basal  joint  of  the  antenna, 
(5)  the  large  complicated  muscles  of  the  abdomen,  (6)  the  ventral 
nerve  cord  communicating  with  the  brain. 

(i)  In  a  side  view  of  a  grasshopper  (Schistocerca)  show  in  the  body: 
head,  prothorax,  mesothorax,  metathorax,  abdomen.  The 
appendages  are;  antennas,  lab  rum,  maxillae,  with  maxillary  palps, 
labium  with  labial  palps,  legs.  The  parts  of  a  leg  are;  coxa, 
femur,  tibia  and  tarsus. 

(j)  Remove  the  wings  and  draw  the  thorax  and  abdomen  on  a  larger 
scale.  Note  especially  the  tympanum  and  the  ten  spiracles — 
eight  on  the  abdomen  and  two  on  the  thorax.  Note  the  number 
of  segments  in  the  abdomen  and  compare  male  and  female. 

(k)   Draw  both  wings  of  one  side  expanded. 


122  ANIMALS 

(1)  In  an  anterior  view  of  the  head  show  the  compound  eyes,  ocelli, 

antennae.     Raise  the  labrum  to  expose  the  mandibles, 
(m)  Compare  Schistocerca  with  Cambarus. 
(n)  Study  a  wasp  as  above  i — 1. 
98.     Vertebra  ta: 

(a)  Draw  a  dorsal  view  of  a  fish  (Perca)  to  show  type  of  symmetry. 
Label  as  in  b. 

(b)  In  a  lateral  view  note: 

1.  Head  with  mouth,  nostrils,  eyes  (eyelids?),  ears  (?),  operculum. 

2.  Body  with  dorsal  and  anal  fins  (count  the  number  of  spines 
and  soft  rays)   and  paired  pectoral  and  pelvic   fins.     The 
lateral  line. 

3.  Tail  and  tail  fin. 

(c)  Note  the  arrangement  of  the  scales.     Remove  some  and  study 
with  the  lens. 

(d)  Study  the  texture  of  the  skin. 

(e)  In  a  specimen  from  which  the  operculum   has   been  removed 
study  the  gills.     Note  the  gill  arches  and  the  gill  rakers  and 
the  gills.      Compare  this  respiratory  system  with  that  of  the 
crawfish. 

(f)  -In  a  median  longitudinal  section  of  the  lamprey  (Petromyzon) 

note  especially  the  notochord,  also  the  brain  and  spinal  cord, 
the  mouth,  pharynx  and  gill  slits,  oesophagus,  stomach  (?), 
intestine,  liver,  heart,  kidney,  gonad  and  gonoduct. 

(g)  In  a  lateral  view  of  the  entire  skeleton  of  any  mammal  show : 

1.  Skull  and  mandible. 

2.  Spinal  column  divided  into  cervical,  dorsal,  lumbar,  sacral 
and  caudal  regions.     Count  the  number  of  vertebrae  in  each 
region. 

3.  Ribs  (number?)  and  sternum. 

4.  Girdles. 

A.  The  pectoral  girdle  consisting  of  a  scapula  and  a  clavicle 

on  each  side. 
P.  The  pelvic  girdle  consisting  of  ilium,  pubis  and  ischium 

on  each  side. 

5.  The  appendages: 

A.  Humerus,  radius  and  ulna,  carpals,  metacarpals,  phalanges 

(how  many?)  digits  (how  many?). 
P.  Femur,  tibia  and  fibula,  tarsals,  metatarsals,  phalanges 

(how  many?)  digits  (how  many?). 


LABORATORY   EXERCISES  123 

(h)  Draw  both  ventral  and  lateral  views  of  a  skull.     Show  the  sutures 

and  identify  the  bones, 
(i)  Draw  lateral,  dorsal  and  anterior  views  of  a  dorsal  or  lumbar 

vertebra  to  show  centrum,  neural  arch,  dorsal  spine,  transverse 

processes  and  articulating  processes, 
(j)  Draw  a  cross  section  of  bone  from  a  prepared  slide.     Note  the 

Haversian  canals,  the  lamellae,  the  lacunae  and  canaliculi. 
(k)  In  the  leg  of  a  frog  from  which  the  skin  has  been  removed  note 

how  the  fleshy  mass  is  composed  of  distinct  muscles.     Note  also 

the  tendons  and  the  relation  of  muscle  to  bone.     Between  the 

muscles  may  be  found  nerves  and  blood  vessels. 
(1)  Study  cross  striped  muscle  fibres  in  a  prepared  slide, 
(m)   In  a  median  longitudinal   section  of  a  dog  fish   (Galeus  or 

Mustelus)  study  carefully  the  brain  and  spinal  cord.     Note  also 

the  vertebral  column  and  compare   the  other   organs   of   the 

body  with  those  of  lamprey  as  in  f. 
(n)  In  another  specimen  which  has  been  dissected  to  show  the 

cranial  nerves  and  brain  identify  the  following: 

1.  Brain:  Olfactory  lobes,   cerebrum,   optic  lobes,  cerebellum, 
medulla  ollongata. 

2.  Cranial  nerves:  I  Olfactory,  II  Optic,  III  Oculomotor,  IV 
Trochlearis  (slender),  V  Trigeminal,  VI  Abducens  (slender), 
VII  Facial,  VIII  Auditory,  IX  Glossopharyngeal,  X  Vagus. 

3.  Spinal  cord  and  spinal  nerves. 

(o)  Draw  dorsal  and  lateral  views  of  the  brain  of  a  mammal  showing 

cerebral  hemispheres,  cerebellum,  and  medulla, 
(p)  Slit  the  skin  of  a  frog  along  the  mid-dorsal  line,  lift  the  skin  of 

one  side  and  note  the  median  dorsal  cutaneous  nerves  passing 

out  to  the  skin, 
(q)  With  a  lens  search  the  inner  surface  of  a  piece  of  skin  of  a  frog  and 

observe  the  white  nerves,  the  veins  and  the  arteries,  the  three 

often  running  parallel, 
(r)  Study  the  digestive  system  of  a  frog  or  turtle.     Note :  oesophagus, 

stomach,   small  intestine,  liver,  gall  bladder,  pancreas,   large 

intestine.     Note  also  the  mesentery  and  the  spleen, 
(s)  Study  as  in  r  the  digestive  system  of  some  mammal, 
(t)  On  the  surface  of  a  tongue  (mammal)  find  the  circumvallate  and 

fungiform  papillae.     If  present  note  also  the  character  of  the 

glottis  and  epiglottis. 


1 24  ANIMALS 

(u)   Study   the   internal   surfaces   of   a   mammalian   stomach   and 

intestine, 
(v)  The  heart  of  a  mammal.     Sketch  the  organ  as  a  whole  showing 

auricles,  ventricles  and  the  aortic  arch, 
(w)  In  a  freshly  killed  turtle  observe  the  beat  of  the  heart  noting  the 

order  of  the  beat  in  auricle's  and  ventricle, 
(x)  Observe  the  circulation  of  the  blood  in  the  tail  of  a  tadpole,  the 

web  of  a  frog's  foot  or  the  gills  of  a  larval  amphibian, 
(y)  In  an  injected  frog  trace  the  principal  arteries,  viz:  The  truncus 

arteriosus  which  divides  into  three  arches: 

1.  The  Carotid  Arch  with  its  branches. 

(a)  The  external  carotid. 

(b)  The  internal  carotid. 

2.  The  Systemic  Arch  with  its  branches. 

(a)  The  subclavian. 

(b)  The  dorsal  aorta  from  which  arise: 

1.  The  cceliaco-mesenteric. 

2.  The  urinogenital. 

3.  The  iliac. 

3.  The  Pulmo-cutaneous  with  its  branches. 

(a)  The  pulmonary. 

(b)  The  cutaneous. 

(z)    An  injected  mammal  may  be  studied  as  in  y. 

(a')  Study  the  lungs  of  a  frog  or  turtle  from  which  the  liver  and 
stomach  have  been  removed.  Trace  the  trachea  and  bronchi 
from  the  glottis  to  the  lungs.  Study  the  internal  structure  of  a 
lung  which  has  been  laid  open. 

(b')  In  preparations  of  a  mammalian  lung  note  the  structure  of  the 
trachea,  the  division  of  the  lung  into  lobes  and  the  internal 
structure  of  a  lung. 

(c')  In  a  male  frog  from  which  all  other  organs  have  been  removed 
observe  the  testes,  the  kidneys  and  the  ureters. 

(d')  In  a  female  frog  observe  the  ovaries  and  the  oviducts. 

(e')  Compare  a  mammal  (rat)  with  the  frog  with  regard  to  the  excre- 
tory and  ^reproductive  organs. 

CLASSES  OF  ANIMALS 

99.    The   following  outlines  may  be  used  to   extend  the  laboratory 
studies  to  some  of  the  other  more  important  phyla  and  classes. 


LABORATORY   EXERCISES  1 25 

100.  Porifera  (Sponges): 

(a)  Study  a  simple  sponge  like  Grantia.     Note  the  general  form, 
the  point  of  attachment,  the  large  excurrent  opening  or  osculum 
and  the  spicules. 

(b)  In  a  dry  specimen  of  Grantia  cut  longitudinally,  note  the  central 
cavity  or  cloaca  and  the  radial  canals.     Note  also  the  form  and 
arrangement  of  all  spicules. 

(c)  A  longitudinal  section  treated  with  acid  to  remove  the  spicules 
and  then  stained  and  mounted  will  show  the  relation  of  the 
radial  canals  and  the  incurrent  canals  or  interradial  spaces. 

(d)  A  dry  specimen  in  cross  section  should  be  studied  in  connection 
with  b. 

(e)  A  cross  section  with  the  spicules,  stained  and  mounted,  will 
show  further  details  especially  with  regard  to  the  arrangement 
of  spicules  and  may  also  show  ova  or  embryos. 

(f)  The  spicules  may  be  set  free  by  dissolving  the  fleshy  parts  in 
boiling  potash.     How  many  kinds  of  spicules  are  there? 

101.  Cnidaria: 

(a)  A  sea  anemone  (Metridium).     In  a  lateral  view  note:  the  col- 
umn, the  base,  the  crown  of  tentacles. 

(b)  In  an  oral  view  note  the  mouth  with  the  grooved  lips  and 
siphonoglyph  (one  or  two). 

(c)  In  a  cross  section  through  the  middle  of  the  column  note: 

1.  The  gullet  with  grooves  and  siphonoglyphes. 

2.  The  gastro-vascular  cavity  incompletely  divided  into  cham- 
bers by  the  mesenteries. 

3.  The  mesenteries  are  of  two  kinds,  complete  and  incomplete 
and  on  their  edges  may  be  found  the  mesenterial  filaments 
and  acontia  and  the  gonads. 

(d)  Study  a  fragment  of  a  coral  (Astrangia).     The  cups  (theca)  each 
contain  a  central  columella  and  a  number  (?)  of  radial  septa. 

1 02.  Platyhelminthes: 

(a)  In  a  living  planarian  note  the  method  of  locomotion,  the  eye 
spots,  the  proboscis. 

(b)  In  an  adult  liver  fluke  note  the  terminal  mouth  and  the  ventral 
sucker. 

(c)  In  a  tape  worm  note: 

1.  The  scolex  with  a  circlet  of  hooks  and  suckers. 

2.  The  body  of  proglottides.     Note  the  form  of  a  proglottis  in 
different  regions  of  the  body. 


126  ANIMALS 

(d)  A  proglottis  cleared  and  mounted  may  show  the  reproductive 
organs,  viz:  ovary,  shell  gland,  vitelline  glands,  uterus,  testes 
and  genital  pore. 

103.  Aschelminthes : 

(a)  Note  the  form  and  movements  of  a  living  "vinegar  eel"  or  a 
thread  worm  from  an  aquarium. 

(b)  In  Ascaris  note  the  form  of  the  body,  the  terminal  mouth  with 
its  lips,  and  the  anus.     If  the  body  is  slit  open  the  intestine  and 
reproductive  organs  may  be  identified. 

104.  Annelida:  Study  a  living  leech  in  water.     Note  its  method  of  swim- 
ming, and  locomotion  by  means  of  its  suckers. 

105.  Echinodermata: 

(a)  In  an  aboral  view  of  a  starfish  (Asterias)  note  the  type  of 
symmetry,  the  central  disc  and  the  arms  or  rays.     On  the 
general  surface  will  be  found  the  hard  spines  and  soft  papulae; 
on  the  disc  the  madreporic  plate. 

(b)  If  a  living  specimen  can  be  had  study  it  in  sea  water  for  the 
method  of  progression. 

(c)  On   the  oral  surface  are  the  mouth,  the  ambulacral  grooves 
with  the  ambulacral  feet,  the  radial  nerve  ridge  in  the  middle 
of  each  ray  and,  at  the  tip  of  each  arm,  a  tentacle  and  eye  spot. 

(d)  If  the  aboral  wall  is  removed  the  stomach  and  hepatic  caeca 
come  into  view  and  beneath  these  the  gonads  and  the  ampullae 
of  the  ambulacral  system.     Note  also  the  structure  of  the  skel- 
etal system. 

(e)  Compare  a  sea-urchin  with  a  starfish. 

(f)  Compare  a  sea-cucumber  with  a  starfish. 

106.  Arthropoda: 

(a)  Compare  a  crab  with  the  crawfish. 

(b)  Observe  some  living  fresh  water  Entomostraca  with  the  micro- 
scope. 

(c)  Study  a  "thousand-leg"  or  centipede. 

(d)  Study  a  spider. 

(e)  For  further  studies  on  insects  consult  a  work  on  entomology. 

107.  Mollusca: 

(a)  Study  the  method  of  locomotion  of  a  common  snail.    Note 
symmetry  of  body  and  shell. 

(b)  In  a  right  lateral  view  note  the  head,  the  foot,  the  collar 
and  shell.    Note  also  the  mouth,  the  tentacles,  the  eyes,  the 
respiratory  opening. 


LABORATORY   EXERCISES  127 

(c)  In  a  small  living  snail  the  movement  of  the  heart  may  be  seen 
through  the  shell. 

(d)  When  the  shell  is  removed  the  lung  chamber  may  be  laid  open. 
Note  also  the  heart,  kidney,  liver,  coils  of  the  intestine  and,  at 
the  top  of  the  spiral  mass,  the  gonad. 

(e)  If  the  dorsal  body  wall  is  removed  from  the  tentacle  to  the  heart 
the  following  organs  come  to  view:  The  buccal  mass,  the  ces- 
phagus  surrounded  by  the  nerve  collar,  the  crop  with  the  sali- 
vary glands  at  either  side,  the  complicated  reproductive  organs 
lying  on  the  right  side  of  the  body. 

(f)  Draw  a  dorsal  view  of  a  clam.     Note  the  symmetry.     (The 
hinge  is  dorsal  and  the  siphon  posterior.) 

(g)  The  clam  may  be  studied  with  one  valve  of  the  shell  removed. 
Note:  The  mantle,  the  two  adductor  muscles  and  the  siphon 
with  its  two  openings.     At  the  dorsal  edge  of  the  mantle  is  the 
pericardial  cavity  in  which  lies  the  heart. 

(h)  Raise  the  mantle  and  observe  the  large  chamber  in  which  lie 
the  visceral  mass  and  the  fleshy  foot.  Upon  the  visceral  mass 
lie  the  two  gills  and  at  the  anterior  edge  the  palps  which  hide 
the  mouth. 

(i)  Draw  a  lateral  view  of  a  squid.  Note  the  head  with  the  arms 
and  eyes.  Behind  the  head  are  the  collar  and  funnel.  The 
body  is  covered  with  a  very  thick  mantle  which  is  expanded 
at  the  end  into  a  fin. 

(j)  Study  the  suckers  on  the  arms.     Find  the  mouth  and  jaws. 

(k)  The  visceral  mass  and  the  gills  are  exposed  by  slitting  open  the 
mantle  on  the  ventral  side. 

(1)  The  rudimentary  shell  is  embedded  in  the  dorsal  surface  of  the 
mantle. 

INTRODUCTION 

302.  Animals  and  Plants.— Animals  present  a  much  greater 
variety  of  types  of  organization  than  plants.  This  is  largely 
because  the  higher  animals  are  vastly  more  complex  than  the 
higher  plants.  The  apparent  complexity  of  a  tree,  for  example, 
is  in  reality  due  chiefly  to  a  repetition  of  similar  parts;  but  in 
animals  there  is  a  progressive  differentiation  of  parts  from  the 
lowest  to  the  highest  forms,  so  that  even  a  single  cell  may  have 


128  ANIMALS 

a  structure  and  function  not  duplicated  by  any  other  cell  in  the 
body.  The  gap  between  the  simplest  and  most  complex 
animals  is  occupied  by  many  types  of  intermediate  degrees  of 
complexity  and  we  shall  therefore  keep  in  mind  several  of  the 
most  significant  of  these  types  while  seeking  to  obtain  a  concep- 
tion of  what  constitutes  an  animal. 

303.  Animal  Types. — As  an  example  of  the  very  simplest 
kinds  of  animals  we  shall  frequently  refer  to  the  amoeba,  a 
minute  speck  of  living  jelly,  quite  common  in  the  bottom  slime 
of  ponds. 

304.  As  a  slightly  more  complex  form  we  will  take  hydra, 
which  is  also  found  in  fresh-water  ponds,  attached  to  plants 
or  other  objects  in  the  water.     It  is  vase-like  in  form  and  has  a 
circle  of  long  slender  arms  or  tentacles  near  the  oral  end. 

305.  As    a    still   more   complex   form   the  common   earth- 
worm may  serve  very  well,  or  a  segmented  marine  worm,  like 
nereis. 

306.  The   crayfish   will  form  another  step  forward.     This 
animal  is  also  common  in  most  parts  of  the  world  and  should 
be  familiar  to  everyone.     In  this  connection  reference  will 
occasionally  be  made  to  insects,  which  belong  to  the  same 
phylum. 

307.  As    an    example    of    the    most    complicated    type    of 
animal  organization  any  mammal  may  be  kept  in  mind,  such 
as  the  cat,  dog  or  rabbit,  or,  better  still,  man.     The  student 
will  be  supposed  to  have  some  knowledge  of  human  anatomy 
and  physiology.     Reference  will  also  be  made  to  other  members 
of  the  vertebrate  phylum,  such  as  fishes,  frogs,  reptiles  and  birds. 

308.  Color  and  Form. — If  we  compare  plants  and  animals 
with  regard  to  color  and  form  we  find  nothing  in  common. 
Animals  contain  no  chlorophyl  and  they  are  therefore  physio- 
logically dependent  upon  plants.     Nor  do  they  have  any  other 
general  color  characteristic.     The  form  of  plants  we  found  was 
determined  by  the  necessity  of  exposing  chlorophyll  tissue  to 


FORM   OF    THE   BODY  I2Q 

the  light,  and  for  maximum  efficiency  this  demands  a  branching, 
or  what  may  be  called  a  diffuse  form  of  body.  Since  animals 
do  not  demand  such  light  exposure  the  branching  form  of  body 
is  also  not  necessary.  As  a  matter  of  fact  the  animal  body  is 
not  only  not  diffuse,  it  is  constructed  in  the  most  compact 
manner  possible.  The  reason  for  this  is,  of  course,  not  far  to 
seek.  It  is  demanded  by  the  most  distinctively  animal  char- 
acteristic— locomotion.  For  the  purposes  of  respiration  animals 
also  require  a  large  exposure  of  surface  to  the  surrounding 
medium,  but  this  is  secured  in  the  gills  and  lungs  by  folding 
surfaces  in  such  a  way  as  to  make  the  respiratory  organs  occupy 
very  little  space  in  proportion  to  the  surface  which  they  expose. 
This  of  course  secures  protection  to  the  organs  but  at  the  same 
time  it  also  allows  greater  freedom  of  motion.  That  the  latter 
is  an  important  consideration  is  evidenced  by  the  fact  that  many 
fixed  animals  are  also  diffuse  in  form. 

309.  Locomotion. — The  ability  to  move  from  place  to  place 
is  the  most  conspicuous  animal  character,  but  coordinate  with 
it  and  inseparably  connected  with  it  is  sensibility  to  external 
influences.     This  latter  character  is  not  wanting  in  plants  but 
it  is  so  much  more  greatly  developed  in  animals  as  to  amount 
practically  to  a  different  thing.     Locomotion  and  sensibility 
go  hand  in  hand  because  locomotion  without  sensibility  would 
be  aimless  and  sensibility  without  the  power  of  motion  would 
be  without  value.     In  this  connection  " motion"   or  " loco- 
motion" must  be  understood  in  a  broad  sense  as  a  muscular 
response  which  may  involve  only  a  part  of  the  body.    The  power 
of  locomotion  carries  with  it  a  large  train  of  interesting  conse- 
quences which  determine  the  form  and  structure  of  the  animal 
even  to   the  minutest  detail.     These  will  be   considered   at 
various  points  as  the  subject  develops,  but  here  we  will  examine 
only  into  the  matter  of  the  external  form  as  resulting  from 
locomotion. 

310.  Axis  of  Locomotion. — In  the  more  primitive  condition, 


130  ANIMALS 

differentiation  results  in  a  repetition  of  similar  parts  and  these 
parts  must  either  be  arranged  radially  or  serially.  But  the 
serial  arrangement  results  in  an  elongated  body  and  this  is 
better  adapted  for  free  locomotion.  Consequently  the  body 
of  the  typical  animal  is  elongated  in  the  axis  of  locomotion. 

311.  Cephalization. — Since    locomotion    is    generally    in    a 
horizontal  direction  the  elongation  of  the  body  is  horizontal. 
But  the  two  poles  of  this  body  are  not  alike,  because  the  prin- 
ciple of  division  of  labor  and  efficiency  would  make  locomotion 
in  one  of  the  two  directions  become  the  principal  direction  of 
locomotion.     The  animal  usually  moves  with  the  same  end 
forward  and  this  end,  which  is  called  anterior,  is  very  different 
from  the  opposite  or  posterior  end.     The  difference  is  chiefly 
due  to  the  development  of  special  sense  organs  at  the  anterior 
end,  because  this  end  comes  more  positively  into  relation  with 
the  forces  which  affect  the  senses.     The  development  of  the  spe- 
cial sense  organs  further  carries  with  it  the  special  development 
of  the  central  nervous  system  of  that  region;  that  is,  the  devel- 
opment of  a  brain.     The  locomotion  of  the  animal  has  to  do 
largely  with  obtaining  food  and  this  probably  determines  that 
the  anterior  end  is  located  near  the  mouth.     Then  the  develop- 
ment of  organs  for  ingestion  and  comminuting  food,  and  the 
sense  organs  connected  with  this  function  still  further  differen- 
tiate the  anterior  end  from  the  posterior.     The  development  of 
all  these  organs  at  the  anterior  end  of  the  animal  forming  a  com- 
plex of  organs  called  the  head  is  called  cephalization.     The 
posterior  end  of  the  body  is  sometimes  developed  into  an  organ 
of  propulsion  or  otherwise  specialized,  but  never  to  the  degree  to 
which  the  more  positive  conditions  bring  the  development  of 
the  anterior  end.     In  this  way  are  determined  the  elongation 
of  the  animal  with  its  principal  axis  horizontal,  and  the  differ- 
entiation of  the  two  poles  into  anterior  and  posterior. 

312.  Dorsal  and  Ventral. — For  animals  which  pass  from  one 
medium  to  another,  as  from  water  to  dry  land  or  from  the  latter 


FORM   OF   THE   BODY 


into  the  air,  two  sets  of  locomotor  organs  might  be  necessary, 
but  in  general  the  principle  of  economy  determines  that  only 
one  set  of  appendages  is  developed.  For  those  forms  which 
move  on  the  bottom  of  the  sea  or  on  land  the  locomotor  ap- 


FIG.  62. — Diagram  of  bilateral  symmetry  (fish),  d-v,  Dorso- ventral  axis; 
r-l,  right-left  axis;  ap,  appendages;  b.c.,  body  cavity;  ch.,  notochord;  d.f.,  dorsal 
fin;  g,  intestine;  h,  heart;  h.a.,  haemal  arch;  m,  muscles;  n.a.,  neural  arch;  sp, 
spinal  cord;  v.c.,  vertebral  column.  (From  Galloway.) 

pendages  will  necessarily  be  constructed  with  reference  to  the 
force  of  gravity.  Since  this  force  operates  in  one  direction 
only,  the  appendages  have  a  one-sided  relation  to  the  body. 
There  are  therefore  an  upper  and  a  lower  side  of  the  body,  and 
these  two  sides  are  not  alike.  Moreover,  since  light,  which 


132  ANIMALS 

also  affects  the  organism,  impinges  more  strongly  from  above, 
it  will  also  operate  to  differentiate  the  upper  and  lower  sides. 
These  two  sides  are  distinguished  as  dorsal  and  ventral, 
respectively. 

313.  Fishes  which  do  not  rest  on  the  bottom  but  always 
float  suspended  in  the  water  do  not  present  the  same  degree 
of  dorso-ventral  differentiation.     In  this  case  the    action   of 
gravity  is  practically  eliminated  by  the  buoyancy  of  the  water. 

314.  Right  and  Left. — With  the  differentiation  of  anterior 
and  posterior  and  of  dorsal  and  ventral  the  animal  comes  to 
have  a  right  side  and  a  left  side.     These  two  sides  are  so  related 
to  the  external  world  that  every  force  which  acts  on  one  side 
affects  the  other  side  also  in  a  symmetrical  way.     In  con- 
sequence, the  two  sides  are  also  symmetrical  in  form  in  every 
way. 

315.  Bilateral  Symmetry. — An  organism  like  the  one  just 
described  is  divided  into  right  and  left  symmetrical  halves  by 
the  vertical  plane  in  which  the  principal  axis  lies.     No  other 
symmetrical   division   of   such   a   form  is   possible.     A   body 
having  such  a  form  is  said  to  be  bilaterally  symmetrical,  and 
this  is  the  type  of  symmetry  found  in  most  animals  and  gener- 
ally in  those  having  marked  freedom  of  locomotion. 

316.  Radial  Symmetry. — Those  animals  which  are  very  slug- 
gish in  movement  or  actually  fixed,  show  little  or  no  evidence  oi 
bilateral  symmetry.     The  principal  axis  is  perpendicular  to  the 
substratum,  and  its  two  poles  are  differentiated;  the  mouth  and 
associated  organs  for  ingestion  are  at  the  free,  oral,  pole,  while 
the  opposite,  aboral,  pole  is  modified  for  attachment.     If  the 
animal  is  not  actually  fixed,  the  oral  pole  may  be  toward  the 
substratum.     In  either  case,  the  organization  of  the  animal  is 
more  or  less  perfectly  radial  with  respect  to  the  principal  axis, 
the  number  of  rays  being  2,  4,  6  or  5  or  a  multiple  of  one  oi 
these  numbers.     The  oral-aboral  differentiation  in  part  cor- 
responds to  the  dorso-ventral  differentiation  of  bilateral  forms. 


FORM   OF   THE   BODY 


133 


The  radial  symmetry  is  to  be  referred  to  the  radial  action  of  the 
environment,  which  is  the  same  in  all  directions  at  right  angles 
to  the  principal  axis.  This  type  of  symmetry  is  characteristic 
of  plants,  and  inasmuch  as  these  radial  animals  approach  plants 
in  their  life  habit,  they  are  affected  by  their  environment  like 
plants  and  consequently  approach  plants  in  their  structure. 
A  b 

B  ab.  o. 


FIG.  63. — Diagram  of  radial  symmetry  as  represented  by  a  medusa, 
view;  B,  lateral  view;  o-ab.o.,  principal  axis. 


a' 


A,  Oral 


317.  Universal  Symmetry. — In  case  external  forces  are  the 
same  in  all  directions  the  corresponding  response  form  would  be 
a  sphere.     This  might  be  called  universal  symmetry.     Such  a 
condition  is  actually  approached  only  by  a  few  protozoa  which 
float  in  the  water,  unattached,  and  are  continually  turning  over 
and  over. 

318.  Asymmetry. — Varying  degrees  of  asymmetry  are  found 
among  fixed  or  sluggish  types.     This  is  more  often  true  of 
colonial  forms  which  grow  by  a  process  of  budding  and  become 
asymmetrical  by  unequal  growth.     In  these  cases,  however,  the 
individual  may  be  perfectly  radial. 

319.  A  study  of  some  exceptional  cases  will  serve  to  "prove 
the  rule."     The  shell  of  gasteropod  molluscs  is  asymmetrical. 
However,  when  the  animal  is  completely  withdrawn  into  the 
shell  and  is  then  completely  asymmetrical,  it  is  also  to  all  in- 


ANIMALS 

tents,  so  far  as  external  forces  are  concerned,  an  inert  body  and 
has  lost  its  animal  character  completely.  When  expanded  and 
moving,  on  the  other  hand,  the  animal  character  reappears  and 
at  the  same  time  the  form  of  the  animal  becomes  largely  or 
completely  bilaterally  symmetrical. 

320.  The  free  swimming  larvae  of  the  Echinoderms  are  per- 
fectly bilateral,  but  when  they  assume  the  less  active  or  fixed 
life  habit  of  the  adult,  they  become  radial  in  symmetry.  This 
change  involves  a  radical  metamorphosis.  In  the  case  of  some 


FIG.  64. — The  bilaterally  symmetrical  free  swimming  larva  of  an  Echinoderr 
(From  Ziegler's  models.) 

of  the  Holothuria  a  second  change  occurs,  in  which  the  radii 
symmetry  is  largely  superseded  by  a  secondary  bilateral  sym- 
metry.    This  is  brought  about  by  the  habit  of  the  animal 
assuming  a  horizontal  instead  of  a  vertical  position  of  th< 
principal  axis. 

321.  The  adult  ascidians  and  barnacles  also  show  a  stroi 
tendency  toward  radial  symmetry,  although  the  active  lam 
are  bilateral. 

322.  A  striking  example  of  a  different  type  is  offered  by  th< 
"flat  fishes,"  such  as  the  flounder  and  sole.     The  young 
these  fishes  have  the  ordinary  type  of  bilateral  symmetry,  am 
in  swimming  they  also  assume  the  erect  position  characteristi< 


FORM   OF   THE  BODY  135 

of  fishes.  But  they  soon  turn  on  one  side  and  in  the  adult 
continue  in  this  attitude,  with  one  side  toward  the  earth.  In 
this  case  the  principal  axis  is  maintained  in  the  same  relative 
position,  but  the  dorso-ventral  and  right-left  axes  are  trans- 
posed in  space.  In  response,  the  form  of  the  animal  also  under- 


FIG.  65. — A  sea-urchin  (Clypeaster)  in  which  a  secondary  bilateral  symmetry 
is  impressed  on  a  radial  organism.     Oral  view,  slightly  reduced. 


goes  a  change,  so  that  the  dorsal  and  ventral  surfaces  become 
symmetrical  and  the  right  and  left  sides  unsymmetrical.  The 
plane  of  symmetry  is  thus  revolved  90°  on  the  principal  axis. 
It  would  be  more  correct,  however,  to  say  that  while  the  animal 


136  ANIMALS 

revolves  90°  on  its  principal  axis,  its  plane  of  symmetry  remains 
fixed.  That  is,  of  course,  what  one  should  expect  following  the 
general  principles  already  laid  down;  for  since  there  is  no  change 
in  the  external  forces  which  cause  symmetry  in  the  organism 
there  should  be  no  change  in  the  plane  of  symmetry  following 
the  revolution  of  the  animal  on  its  axis. 


FIG.  66. — The  flounder,  Pseudopleuronectes  Americanus,  showing  approximate 
dorso-ventral  symmetry.  Note  that  both  eyes  are  on  the  right  side  of  the  head. 
(From  Hegner,  after  Goode.) 


323.  Size  and  Differentiation. — Animals  vary  greatly  in  size 
and  complexity  of  structure,  from  the  microscopic  protozoa  to 
the  gigantic  mammals,  and  it  is  of  interest  to  note  that,  in  a 
general  way,  size  and  complexity  vary  together.  Superiority 
in  size  is  of  itself  an  advantage,  especially  where  there  is  a 
contest  between  individuals.  But  more  important  is  the  advan- 
tage derived  from  complexity,  which  permits  of  differentiation, 
division  of  labor  and  consequent  efficiency.  Considerable 
differentiation  may  be  found  between  the  parts  of  the  same  cell, 
as  in  the  protozoa,  but  when  the  body  is  composed  of  many 
cells  the  differentiation  may  be  vastly  greater,  both  in  regard  to 
the  number  of  kinds  of  differentiation  and  the  degree  to  which 
it  is  carried.  Thus  the  functions  of  contraction,  irritability, 


ORGANIZATION  137 

digestion,  etc.,  may  be  taken  up  by  the  different  cells  or  groups 
of  cells  and  performed  more  efficiently. 

The  various  kinds  of  differentiation '  may  be  grouped  under 
the  following  heads : 

324.  Integument. — An  extremely  important  set  of  organs 
are  the  various  protective  structures  which  cover  the  entire 
surface  of  the  body  of  all  but  the  very  lowest  animals.     The 
most  important  of  these  structures  are  the  hair,  feathers,  scales, 
bony  plates,  cuticular  secretions,  shells,  glands  and  the  un- 
modified skin  itself.     These  together  comprise  the  integument. 

325.  The  Nerve-Muscle  Mechanism. — The  nervous  and  the 
muscular  tissues  are  the  most  highly  differentiated  tissues  of 
the  body.     For  efficiency  the  sense  organs  must  be  very  numer- 
ous, so  that  on  the  surface  of  the  body  there  is  scarcely  a  point 
large  enough  to  be  visible  which  is  not  occupied.     The  muscles 
for  strength  must  be  massive.     The  central  nervous   system, 
through  which  all  the  various  organs  of  the  body  are  brought 
into  harmonious  relations,  especially  the  sense  organs  and  the 
organs  of  response,  is  the  most  complicated  organ  of  the  body 
and  is  also  of  considerable  size.     The  weight  of  a  large  body 
requires  special  supporting  structures,  and  for  the  most  efficient 
application  of  muscular  energy  for  locomotion,  a  system  of 
levers  is  necessary.     These  structures  comprise  the  skeleton 
and  the  connective  tissues,  which  together  constitute  the  largest 
set  of  organs  in  the  body. 

326.  Digestion. — The  highly  differentiated  tissues  just  de- 
scribed have  surrendered  the  function  of  digestion  to  other  cells 
of  the  body,  and  these  are  connected  with  the  central  cavity, 
in  which  digestion  takes  place.     The  various  phases  of  the 
digestive  process   are   separated   and   distributed   to  distinct 
groups  of  cells,  composing  as  many  organs.     These  together 
constitute  the  digestive  system. 

327.  Circulation. — The   digested  food  is  absorbed  by  the 
digestive  tract  in  much  larger  quantities  than  is  necessary  to 


138  ANIMALS 

i 

nourish  the  tissues  of  the  digestive  tract  itself.  It  is  in  a  fluid 
state  as  blood  plasma  and  is  available  for  assimilation  by  the 
other  tissues  of  the  body.  But  a  large  part  of  these  tissues  is 
too  far  removed  from  the  digestive  tract  to  be  nourished  by 
transfusion.  A  system  of  vessels  becomes  necessary  for  the 
conduction  of  the  blood  plasma  and,  in  addition,  a  heart  to 
force  the  blood  along. 

328.  Respiration. — The   smaller   organisms   absorb    enough 
oxygen  through  the  general  surface  of  the  body  to  supply  the 
needs  of  all  the  tissues.     But  increase  in  size  also  brings  some 
of  the  tissues  too  far  from  the  surface  to  be  supplied  in  this  way. 
Moreover,    the    development    of    impervious    integumentary 
tissues  prevents  the  absorption  of  much  oxygen  through  the 
general    surface.     There    then    becomes    necessary    a    special 
respiratory    organ — gills    or    lungs.       From    the    organs    of 
respiration    the    oxygen    can   reach   the   tissues   through   the 
channels   followed   by   the   blood  plasma,  either  dissolved  in 
the  blood  plasma  or  carried  by  special  vehicles,  the  red  blood 
corpuscles. 

329.  Excretion. — The   waste   products    of    metabolism    are 
voided  by  the  smallest  animals  through  the  general  body  sur- 
face.    But  this  also  becomes  impossible  in  the  higher  animals, 
where  they  are  taken  up  by  the  blood  and  are  then  in  part  elimi- 
nated through  special  excretory  organs,  the  nephridia,  kidneys, 
etc. 

330.  Reproduction. — Most   of   the   protozoa   reproduce   by 
division  of  the  body,   or  by  budding.     These  methods  also 
occur  largely  among  the  lower  metazoa,  but  highly  differentiated 
tissues  lose  the  power  of  division  and  the  more  complex  animals 
have  not  the  power  of  reproducing  in  this  way.     In  them  there 
is  a  special  organ,  in  which  undifferentiated  tissue  is  reserved 
for  reproduction. 

331.  Organization  of  the  Body. — We  thus  see  that  the  de- 
velopment of  the  animal,  i.  e.,  the  nerve-muscle  mechanism, 


ORGANIZATION  139 

to  the  highest  degree  involves  the  development  of  an  organism 
made  up  of  nine  systems  of  organs: 

1.  THE  INTEGUMENTARY  SYSTEM. 

2.  THE  SENSORY-NERVOUS  SYSTEM. 

3.  THE  MUSCULAR  SYSTEM. 

4.  THE  SKELETAL  SYSTEM. 

5.  THE  DIGESTIVE  SYSTEM. 

6.  THE  CIRCULATORY  SYSTEM. 

7.  THE  RESPIRATORY  SYSTEM. 

8.  THE  EXCRETORY  SYSTEM. 

9.  THE  REPRODUCTIVE  SYSTEM. 

332.  "Higher"  and  "Lower"  Animals. — The  functions  per- 
formed by  these  nine  systems  of  organs  are  all  performed  by 
the  undifferentiated  protoplasm  of  the  amoeba  and  must  like- 
wise be  provided  for  by  every  other  animal.  The  higher  forms 
are,  therefore,  not  distinguished  by  the  development  of  new 
functions,  but  by  the  effectiveness  with  which  these  functions 
are  performed.  Under  favorable  conditions  the  functions 
of  the  amoeba  are  equal  to  the  demands  made  upon  them,  but 
with  a  serious  change  in  these  conditions  they  fail  and  the  amoeba 
comes  to  naught.  Against  drouth,  heat,  cold,  the  lack  of  food 
in  the  immediate  vicinity  and  the  attacks  of  larger  animals 
the  amoeba  has  no  defense.  On  the  other  hand,  a  higher  animal, 
say,  e.  g.,  a  wolf,  is  effectually  protected  against  dessication  by 
his  skin.  Ordinary  climatic  changes  of  temperature  are  auto- 
matically compensated  for  and  the  body  maintains  an  equable 
internal  temperature.  When  food  fails  he  travels  far  in  search 
of  more,  and  when  attacked  he  knows  how  to  defend  himself. 
Indeed,  he  is  a  living  demonstration  of  his  superiority,  for  his 
life  is  maintained  by  the  destruction  of  other  living  things 
which  are  not  able  to  defend  themselves  against  him.  He 
demonstrates  his  superiority,  and  we  habitually  distinguish 
such  capable  forms  from  the  less  capable  by  the  terms  higher 
and  lower.  But  there  is  still  another  way  in  which  the  wolf 


140 


ANIMALS 


demonstrates  his  superiority.  The  life  of  the  amoeba  is  brief 
in  time  as  well  as  circumscribed  in  space;  the  range  of  its  experi- 
ences are  as  limited  as  its  sensibilities  are  vague  and  the  memory 
of  them  instantly  vanishes.  The  wolf,  however,  lives  on  for 

days,  months  and  years.  His 
highly  specialized  nerve  cells  not 
only  feel  infinitely  more  acutely 
but  they  are  able  to  retain  impres- 
sions, and  through  his  compara- 
tively long  life  these  accumulated 
experiences  are  made  to  serve  to 
his  advantage.  The  intelligence 
of  the  wolf  goes  far  to  make  him 
independent  of  his  environment 
and  he  thereby  demonstrates  his 
superiority.  The  highest  animals 
are  those  most  completely  inde- 
pendent of  the  conditions  under 
which  they  live. 

333 .  Segmentation. — Compar- 
ing the  larger  and  smaller  animals 
again  from  another  point  of  view: 
The  greater  size  of  the  body  may 
be  due  to  larger  organs,  or  it  may 
also  result  from  a  repetition  of 
similar  organs,  as  in  plants.  The 
former  condition  is  well  exempli- 
fied by  the  phylum  Mollusca,  in 
which  the  repetition  of  similar 

organs  (in  this  sense)  does  not  occur,  and  yet  one  class  of 
this  phylum  (Cephalopods)  has  attained  a  high  degree  of 
development,  and  counts,  in  some  of  its  species,  animals  of  the 
greatest  size. 

334.  When  repetition  of  organs  occurs  it  may  affect  some 


FIG.  67.— The  anterior  end  of 
an  annelid,  Nereis.  The  body 
consists  of  upward  of  130  similar 
segments  or  metameres.  Hence 
the  animal  is  said  to  be  homon- 
omously  segmented. 


SEGMENTATION  141 

systems  of  organs  more  than  others,  but  usually  there  is  a 
tendency  for  the  various  systems  to  be  repeated  in  the  same 
degree.  In  this  way  the  body  is  divided  into  segments,  each  one 
of  which  contains  a  segment  of  each  system  of  organs.  In 
elongated  animals  the  body  segments  are  arranged  in  a  linear 
series,  and  are  called  metameres.  Within  the  metamere  each 
system  of  organs  is  represented  by  a  single  segment  if  the  system 
is  median  in  position,  or  by  a  symmetrical  pair  if  they  are  lateral. 

335.  The  segments  of  radial  animals  are  called  antimeres 
and  they  are  arranged  radially  about  the  principal  axis  of 
symmetry.     The  antimeres  are  also  bilaterally  symmetrical. 
(Why?) 

336.  Metameric   segmentation   introduces    a   new   type    of 
differentiation,  the  differentiation  of  segments.     In  the  phylum 
Vermes  there  is  little  differentiation  of  segments  and  hence  the 
segments  are  said  to  be  homonymous.     When  the  segments  are 
differentiated  they  are  heteronymous.     This  occurs  in  progres- 
sive stages  through  the  phyla  Arthropoda  and  Vertebrata,  so 
that  in  the  higher  forms   the   segmentation  is   considerably 
obscured.     Of  course,  differentiation  of  segments  greatly  in- 
creases the  complexity  of  organization. 

337.  Segmentation  of  the  body  is  a  means  by  which  its 
flexibility  may  be  provided  for.     This  is  of  special  importance 
in  animals  having  a  skeleton. 

INTEGUMENT 

338.  The  amoeba  is  said  to  be  a  naked  cell,  i.  e.,  it  has  no  cell 
wall,  and  therefore  can  scarcely  be  said  to  have  an  integument. 
The  surface  layer  of  the  protoplasm,  the  pellicle,  is  slightly 
denser  than  that  lying  deeper,  and  its  consistency  is  such  as 
to  maintain  a  well-defined  boundary  between  the  organism  and 
the  surrounding  water.     The  protoplasm  is  so  nearly  the  density 
of  water  and  the  animal  so  minute  that  little  force  is  required 


142 


ANIMALS 


to  maintain  the  integrity  of  the  body.  The  delicate  pellicle 
is  therefore  sufficient  for  the  amoeba,  although  for  larger  forms 
it  would  be  entirely  inadequate. 

339.  The  body  wall  of  hydra  consists  of  two  layers  of  cells, 
an  outer  ectoderm  and  an  inner  entoderm.     The  cells  of  the 


FIG.  68. — Amoeba  Proteus.  Na,  A  cluster  of  algal  cells  which  is  being  engulfed 
by  the  protoplasm  of  the  amceba;  Cr,  contractile  vacuole;  N,  nucleus.  (From 
Marshall,  after  Doflein). 


ectoderm  are  practically  naked  protoplasm  on  the  exposed 
surfaces,  but  this  protoplasm  is  so  dense  that  it  serves  as  an 
integument.  In  the  closely  related  hydroids  the  ectoderm 
secretes  a  thin  membrane  known  as  the  perisarc  or  cuticula. 
It  is  extremely  tough  and  well  adapted  for  protection  and 
support. 

340.  The  epidermis  of  worms  corresponds  to  the  ectoderm 
of  hydra,  but  is  much  thicker  because  of  the  columnar  form  of 


INTEGUMENT 


143 


the  cells  composing  it.  It  secretes  a  very  thick  and  firm  cuticula. 
Both  epidermis  and  cuticula  vary  greatly  in  thickness  in  differ- 
ent species  of  worms,  but  this  variation  corresponds  approxi- 
mately with  the  size  of  the  worm. 


/*•  B 


FIG.  69. — A,  Diagram  of  Hydra;  B,  portion  of  the  wall  highly  magnified; 
6,  bud;  ect.,  ectoderm;  ent,  entoderm;  /,  foot;  //,  flagellum;  g.v.,  gastro-vascular 
cavity;  m,  mouth;  mes,  supporting  lamella;  m.f.,  muscle  fibre  of  the  ectoderm 
cells;  n,  nettling  cells;  n',  same,  exploded;  ««.,  nucleus;  t,  tentacle;  v,  vacuole. 
(From  Galloway.) 

341.  The  integument  of  the  crayfish  is  similar  to  that  of  the 
worm.  The  chief  difference  lies  in  the  much  greater  thickness  of 
the  cuticula,  which  here  consists  of  a  peculiar  substance  called 
chitin.  Chi  tin  is  an  extremely  firm  and  elastic  substance, 


144 


ANIMALS 


FIG.  70. — Diagrams  of  the  integument.     A,  A  sectional  view  of  the  epidermis 
on  the  superior  ligula  of  nereis;  B,  a  surface  view  of  the  same  showing  the  cell. 


INTEGUMENT  145 

which  is  highly  resistant  to  most  chemical  reagents.  The  cu- 
ticulaof  the  crayfish  also  contains  considerable  quantities  of  lime 
salts,  which  add  much  to  its  hardness.  The  integument  of 
insects  is  similar  to  that  of  the  crayfish,  but  the  cuticula  lacks 
the  lime. 

342.  This  type  of  integument  is  extremely  effective  as  a 
protective  structure,  and  it  will  be  noted  that  within  the  group 
of  Arthropods,   we   first   find   animals  which   can  withstand 
exposure  to  dry  air.     A  disadvantage  of  the  arthropod  type  of 
integument  is  that  it  is  too  rigid  to  permit  great  freedom  of 
motion.     For  this  reason  the  integument  of  both  body  and 
appendages  is  divided  into  segments  which  are  connected  by 
zones  of  more  flexible  tissue.     Flexure  of  the  body  and  appen- 
dages can  only  occur  at  these  points.     (See'ecdysis,  page  346.) 

343.  The  skin  of  vertebrates,  especially  that  of  Mammals, 
is  much  more  complex.     The  epidermis  consists  of  many  layers 
of  cells,  which  are  continually  increasing  in  number  by  the  active 
growth  and  division  of  the  deeper  lying  ones.     The  superficial 
layers  are  composed  of  lifeless  cells  which  have  been  transformed 
into  flattened,  horny  scales.     This  horny  layer  takes  the  place 
of  a  cuticula.     In  addition  to  the  epidermis  there  is  a  deeper 
layer,  the  dermis,  which  is  much  thicker  and  consists  chiefly 
of  felted  connective  tissue  fibres. 


outlines;  C,  a  sectional  view  of  the  thick  epidermis  of  the  ventral  side  of  nereis, 
with  the  underlying  circular  and  longitudinal  muscles.  In  A  the  cells  are 
cubical  and  the  cuticula  very  thin;  in  C  the  cells  are  columnar,  the  cuticula  is 
very  thick  and  pierced  by  pores  through  which  glands  open  and  sensory  hairs 
project.  The  lumina  of  the  gland  cells  are  represented  as  light  oval  spaces. 
D,  a  sectional  view  of  the  epidermis  of  the  crayfish;  E,  the  epidermis  of  a  mammal 
with  a  hair  follicle  in  which  a  hair  (h)  is  beginning  to  develop;  F,  the  epidermis 
of  a  mammal  highly  magnified  and  showing  its  layers — horny  layer,  granular 
layer  and  Malpighian  layer;  G,  a  section  through  the  skin  of  a  mammal  including 
epidermis,  dermis,  and  a  hair  follicle;  H,  diagram  of  the  epidermis  of  a  reptile 
with  its  outer  layers  (black)  solidified  into  horny  scles;  /,  a  similar  diagram  of 
the  bony  scales  beneath  the  epidermis  of  a  fish;  /,  a  section  of  the  skin  of  a 
bird  to  show  a  feather  in  an  early  stage  of  development.  Bl.V.,  blood-vessels; 
C.M.,  circular  muscles;  Cu.,  cuticula;  D,  dermis;  Ep,  epidermis;  h,  hair;  H.L., 
horny  layer;  l.m.,  longitudinal  muscles;  M,  muscles;  M.L.,  Malpighian  layer; 
s  (in  G),  sebaceous  glands;  s  (in  H  and  7),  scales;  S.C.,  sensory  cell. 


146  ANIMALS 

344.  This  type  of  integument  is  extremely  flexible  and  there- 
fore does,  not  impede  locomotion  or  other  movements.     At 
the  same  time  it  is  very  tough,  fairly  resistent  to  mechanical 
injury,  and  the  lifeless  superficial  layers  of  the  epidermis  are 
impervious  to  water  and  thus  protect  the  living  parts  from  the 
dry  air. 

345.  In  addition  to  these  undifferentiated  portions  of  the 
various  types  of  integument  there  are  also  certain  important 
specialized  structures  developed  which  serve  as  supplementary 
protective  organs,  as  organs  of  defense  and  offense,  as  prehensile 
organs  and  as  accessory  organs  of  locomotion. 

346.  The  most  common  type  of  differentiation  consists  simply 
of  a  local  thickening  of  the  cuticula  or  epidermis.     Thus  in 
many  worms  minute  tubercles  or  larger  jaw-like  structures  are 
found  on  the  walls  of  the  mouth  and  pharynx.     The  setae  and 
aciculae  on  the  parapodia  and  sometimes  scales  on  the  back,  are 
of  similar  origin.     In  insects  and  Crustacea  the  sensory  hairs 
found  especially  on  the  antennae  and  mouth  parts,  and  around 
the  joints  of  the  appendages  and  body  are  also  produced  by  the 
unusually  active  secretion  of  chitin  by  one-  or  a  few  cells  of  the 
underlying  epidermis. 

347.  In  Vertebrates  the  epidermis  becomes  modified  in  a 
great  variety  of  ways  by  the  aggregation  of  the  minute  horny 
scales  into  exceedingly  firm  structures,  which  serve  a  great  vari- 
ety of  purposes.     Among  the  most  important  of  these  structures 
are  nails,  claws,  hoofs,  spurs,  horns,  beaks,  "  tortoise  shell," 
"  whale-bone,"  scales  of  certain  kinds,  hairs  and  feathers.     The 
scales  found  on  reptiles,  birds  and  some  fishes  are  merely  thick 
and  compact  areas  of  the  corneous  layer  of  the  epidermis.     The 
hair  differs  from  the  scale  only  in  its  form.     The  feather  may  be 
likened  to  a  hair  greatly  enlarged  in  diameter  and  hollow,  with 
certain  parts  of  its  shaft  splitting  in  a  complicated  fashion  and 
thereby  producing  the  vane. 

348.  The  scales  of  most  fishes  are  not  horn  but  thin  plates 


INTEGUMENT 


147 


of  bone  which  are  formed,  not  in  the  epidermis,  but  in  the 
dermis.  In  some  fishes  there  is  deposited  on  the  upper  surface 
of  the  bony  scale  a  layer  of  enamel  which  is  due  to  the  activity 
of  the  cells  of  the  overlying  epidermis.  Teeth  are  also  formed 
in  this  way,  the  dentine  being  merely  a  kind  of  bone.  Antlers 
are  bone  and  are  formed  by  the  dermis. 

349.  Glands. — Glands  form  another  type  of  differentiated 
integumentary  structures.  They  are  of  the  simplest  form  in 
hydra  and  worms  where  they  consist  of  single  cells  of  the 
epidermis.  These  cells  do  not  secrete  a  cuticula  on  their  free 


Cu. 


EP. 


FIG.  71. — Glands.  A,  The  ectoderm  of  hydra  showing  granules  of  cement  (g) 
secreted  by  the  cells  at  the  surface;  B,  a  section  through  the  epidermis  of  nereis, 
showing  a  number  of  unicellular  glands.  Only  the  outlines  of  the  outer  ends  of 
the  gland  cells  are  shown.  The  nuclear  portions  are  below  but  not  distinguish- 
able in  the  figure.  Cu,  Cuticula;  Ep,  epidermis;  g,  pores  of  the  glands;  C, 
diagram  of  a  unicellular  gland  at  the  beginning  of  secretion;  D,  the  same  when 
swollen  with  secretion;  E,  the  same  cell  after  its  contents  are  ejected. 

surfaces,  but  do  secrete  other  substances  which  at  first  accumu- 
late within  the  bodies  of  the  cells  but  are  later  forced  out 
through  the  pores  in  the  cuticula  which  were  formed  by  the  non- 
secretion  of  cuticula  by  the  gland  cells.  The  secretions  accumu- 
late until  the  excess  gradually  oozes  out  through  the  pores  or  is 
suddenly  forced  out  in  larger  quantities  by  the  contraction  of  the 
surrounding  tissues.  In  the  case  of  worms  and  many  other 
aquatic  animals  the  substance  secreted  takes  up  water  and  forms 
slime.  The  function  of  the  slime  is  probably  in  most  cases 


148  ANIMALS 

protective,  but  in  special  cases  it  may  serve  a  variety  of  other 
functions.  Sometimes  it  serves  to  cement  together  the  parti- 
cles of  earth  to  form  a  tube  in  which  the  animal  lives.  Some- 


FIG.  72. — A  parchment-like  tube  constructed  by  a  marine  annelid,  Chsetop- 
teris.  The  tube  is  buried  in  the  mud,  except  an  inch  or  two  of  each  end  which 
project  above  the  surface.  Both  ends  are  open  to  permit  a  current  of  water  to 
pass  through.  X  2/3. 

times  it  is  used  in  locomotion,  as  in  case  of  snails.  The  skin 
of  fishes  is  richly  supplied  with  slime  glands.  Hydra  and 
many  other  animals  attach  themselves  temporarily  or  per- 


GLANDS 


149 


manently  by  a  secretion  which  acts  like  a  cement.     Leathery 
tubes  are  formed  by  many  worms  by  a  secretion  which  hardens 


FIG.   73. — Oven-shaped  shelters  of  the  caddice-worm,  composed  of    pebbles 
fastened  together  with  silk  fibres  spun  from  the  mouth. 


FIG.  74.— Another  type  of  shelter  tubes  of  caddice-worms,  composed  of  sticks 
and  pebbles.  The  caddice-worm  is  the  aquatic  larva  of  the  caddice-flv 
(Trichoptera). 

in  the  water  into  an  exceedingly  tough  fibre.     There  is  often 
considerable  lime  mingled  with  these  secretions  and  sometimes 


150  ANIMALS 


the  lime  is  deposited  in  great  quantities.     This  is  notably  true 
of   the   corals,    which   are   closely   related    to   hydra.     Many 


FIG.  75. — A  common  type  of  massive  coral,  Solenastrea,  from  Bermuda.  Each 
cup  is  formed  by  a  polyp  and  radial  septa  in  the  cups  indicate  the  pairs  of 
mesenteries  of  the  polyp.  Slightly  reduced. 

worms  live  in  hard,  limey  tubes  which  they  secrete.  The  shell 
of  the  mollusc  originates  in  the  same  way,  but  here  it  forms  part 
of  the  body  of  the  animal. 


GLANDS,  SENSE  ORGANS  151 

350.  The  epidermis  of  Crustacea  and  Insects  is  generally 
devoid  of  glands.     Fishes  and  Amphibia  are  well  supplied  with 
slime  glands.     Reptiles  have  practically  no  glands  in  the  skin. 
In  Birds  there  is  one  important  gland  or  group  of  glands.     This 
is  the  uropygal  gland,  located  on  the  tail.     It  secretes  an  oil 
which  is  transferred  by  the  bird  by  means  of  its  beak  to  the 
surface  of  the  feathers  when  preening.     The  oil  keeps  the 
feathers    flexible    and    prevents    wetting.     For    this    reason 
"  water   rolls  off  a  duck's  back/'  as  it  also  does  from  other 
birds. 

351.  The  skin  of  Mammals  is  provided  with  two  kinds  of 
glands,  sebaceous  and  sweat  glands.     The  former  are  grouped 
around  the  hair  follicles  and  their  oily  secretion  escapes  at  the 
base  of  the  hair.     It  not  only  serves  to  keep  the  hair  flexible 
but  also  the  corneous  layer  of  the  epidermis.     This  is  a  very 
important  function,   since   the   dead   tissue   would   otb       :~e 
break  and  expose  the  living  tissue  beneath,  which  at 
happens  in  the  chapping  of  an  abnormally  "dry"  skiii       v*ie 
oil  also  renders  the  skin  impervious  to  water. 

352.  The  sweat  glands  are  the  thermostatic  organs  of  the 
body.     They  will  be  discussed  elsewhere.     (Page  411.) 

SENSE  ORGANS 

353.  Aru'mals  which  live  in  the  water  may  sense  their  food 
in  one  or  more  of  three  ways,  viz. :  Sight,  smell,  and  taste.     For 
amoeba  the  first  is  excluded,  since  amoeba  cannot  be  said  to 
have  a  sense  of  sight.     The  sensations  of  taste  and  smell  are 
both  due  to  chemical  stimuli;  that  is  to  say,  the  substances 
which  stimulate  the  sense  organs  must  be  in  solution,  and  the 
stimulation  itself  is  a  chemical  process.     The  distinction  be- 
tween the  two  sets  of  senses  is  largely  a  matter  of  position  of  the 
sense  organs.     The  organs  of  taste  are  located  in  the  mouth,  while 
those  of  smell  are  elsewhere  on  the  surface  of  the  body,  usually 


152 


ANIMALS 


in  the  vicinity  of  the  mouth.  For  amoeba  no  such  distinction 
is  possible,  and  we  can,  therefore,  only  speak  of  a  chemical 
sense.  Amoeba  has  been  observed  to  engulf  particles  of  sand 
and  other  inorganic  substances  which  could  not  serve  as  food, 
but  in  spite  of  this  fact  there  is  much  evidence  to  show  that  there 
is  a  chemical  sense.  Ordinarily  the  animal  distinguishes  food 
particles  from  others  and  has  even  been  observed  to  follow  a 
moving  protozoan,  which  it  also  captured  and  devoured.  Many 
observations  lead  to  the  general  conclusion  that  the  protozoa 
generally  are  sensitive  to  the  chemical  condition  of  the  sur- 
rounding medium.  They  are  attracted  to  or  repelled  from  the 
source  whence  such  substances  are  diffusing  through  the  water. 
Strong  stimuli  produce  decided  responses,  such  as  a  contraction 
of  the  amoeba  into  a  spherical  mass. 

354.  Amoeba  is  also  sensitive  to  mechanical  stimuli,  such  as  a 
touch  or  a  jar.     By  this  means  it  "feels"  the  presence  of  a 
foreign  object  to  which  it  may  adhere.     If  the  stimulus  is 
irritating  the  response  may  take  the  form  of  a  secretion  of 
slime,  the  withdrawal  of  the  pseudopodium  or  the  complete 
contraction  of  the  amoeba  into  a  spherical  mass. 

355.  Amoeba  is  also  sensitive  to  strong  light  though  not  as 
much  so   as  some  other  colorless  protozoa.     Usually,   when 
other  things  are  equal,  protozoa  may  be  observed  to  seek  a 
point  where  the  light  is  neither  excessively  strong  nor  weak. 

356.  To  changes  in  temperature  amoeba  is  also  sensitive. 
With  increase  in  temperature  it  becomes  more  active,  until  at 
about  35°  C.  it  contracts  and  remains  motionless.     With  a 
falling  temperature  activity  is  also  lowered  until  at  a  little 
above   o°   C.   it   ceases   entirely,   often   without   contraction. 
When  possible  amoeba  will  move  out  of  a  region  of  extremely 
high  or  low  temperature  to  one  more  nearly  normal. 

357.  Though  amoeba  is  sensitive  to  these  various  stimuli, 
yet  it  has  no  sense  organs.     There  is  no  differentiation  of  organs 
for  the  reception  of  the  different  stimuli,  nor  yet  for  the  re- 


SENSE    ORGANS  153 

ception  of  stimuli  in  general.     Sensibility  is  one  of  the  primary 
functions  of  undifferentiated  protoplasm. 

358.  Almost  the  same  may  be  said  of  hydra.     The  organism 
is  sensitive  to  the  same  stimuli  and  to  no  others.     So  far  as  is 
known  there  is  no  localization  of  sensory  function.     All  the 
cells  of  the  body  consist  largely  of  undifferentiated  protoplasm, 
which  is  probably  the  organ  of  general  sense  as  in  amoeba.     It 
is  true  that  some  of  the  cells  of  the  ectoderm  project  beyond  the 
surface  by  slender  protoplasmic  processes  which  are  probably 
organs  for  the  reception  of  stimuli.     Since  these  processes  are 
more  exposed  they  are  more  readily  affected  by  stimuli  and 
hence  may  be  regarded  as  incipient  sense  organs. 

359.  In  some  other  Coelenterates  as,  e.  g.,  the  medusae,  the 
sensory  cells  and  the  nervous  elements  generally,  are  better 
developed.     Many  cells  of  the  ectoderm  are  provided  at  their 
free  surfaces  with  sensory  hairs  while  the  opposite  ends  of  the 
cells  are  prolonged  into  long  fibres  which  extend  for  some 
distance  under  the  ectoderm. 

360.  In   nereis   the   sense   cells   are   clearly   differentiated 
from  the  other  cells  of  the  epidermis.     Each  sense  cell  projects 
through  the  cuticula  by  a  single  protoplasmic  process,  while 
the  remainder  of  the  cell  is  elongated  into  a  fibre  which  extends 
deep  into  the  body  to  the  central  nervous  system.     The  nucleus 
of  the  cell  often  lies  in  the  epidermis,  but  it  may  also  lie  immedi- 
ately beneath  the  epidermis  or  even  at  a  considerable  distance 
beneath  the  surface.     In  the  latter  case  a  number  of  such 
nuclei  may  be  collected  into  a  group  which  constitutes  a  gang- 
lion, and  if  the  fibres  run  parallel  in  a  bundle  they  form  a 
nerve. 

361.  The  sensory  cells  of  Arthropods  are  very  much  like 
those  of  nereis,  but  they  do  not  always  have  the  exposed  proto- 
plasmic terminations.     Instead,  the  fibre  may  end  at  the  base, 
or  in  the  axis  of  one  of  the  cuticular  sensory  hairs  mentioned 
above.     (Page  146.)     The  hairs  serve  mechanically  to  trans- 


154 


ANIMALS 


mit  the  stimulus  to  the  sensory  element  but  are  not  themselves 
sensitive. 

362.  The  sensory  elements  found  in  the  skin  of  Vertebrates 
are  of  various  types  which  may  be  divided  into  two  classes.  In 
one  class  the  fibre  ends  in  a  bushy  system  of  branches  which 
penetrate  among  the  other  normal  elements  of  the  surrounding 
tissues.  In  the  other  class  the  fibre  may  end  with  or  without 


FIG.  76. — The  blue  crab.     View  of  the  region  around  the  mouth  to  show  the 
groups  of  cuticular  hairs,  which  are  in  large  part  sensory.     X  i  1/2. 

terminal  branching,  but  in  either  case  there  are  always  some  of 
the  cells  of  the  surrounding  tissues  modified  to  form  a  special 
stimulating  organ.  The  first  class,  the  free  nerve  terminations, 
are  found  chiefly  in  the  deeper  layers  of  the  epidermis,  though 
they  may  also  be  found  in  the  dermis.  The  second  class,  which 
for  want  of  a  better  name  may  be  called  sensory  corpuscles, 
are  found  chiefly  in  the  superficial  and  deeper  layers  of  the 


SENSE    ORGANS  155 

dermis,  though  some  are  also  found  in  the  epidermis  (in  man 
only  in  the  dermis). 

363.  The  nuclei  of  all  sensory  elements  of  the  skin  of  verte- 
brates lie  in  the  spinal  ganglia  and  homologous  ganglia  of  the 
cranial  nerves. 

364.  The  sense  organs  just  described  are  those  which  are 
generally  distributed  over  the  whole  surface  of  the  body.     The 
senses  to  which  they  correspond  are,  in  man,   touch,   cold, 
warmth  and  pain.     Each  of  these  senses,  with  the  possible 
exception  of  pain,  has  its  own  set  of  sensory  elements,  although 
the  correspondence  between  sense  organs  and  senses  has  not 
yet  been  completely  determined.     How  far  these  senses  are 
differentiated  in  the  lower  animals  is  also  not  known. 

365.  In  the  higher  animals  there  are  also  deeper  lying  sense 
organs,   which  are  located  in  the  sub-cutaneous   connective 
tissue,  in  the  muscles  and  tendons  and  even  in  the  mesentery. 
To  these  organs  are  ascribed  a  sense  of  weight  and  a  sense  of 
position,  or  attitude,  of  the  member  of  the  body  with  regard  to 
the  other  members  of  the  body. 

ORGANS  OF  SPECIAL  SENSE 

366.  Besides  the  general  sense  organs  described  above,  we 
find  in  all  the  higher  animals  special  sense  organs,  which  are 
developed  in  very  limited  regions  of  the  body  and  which  are 
often  very  complex  in  structure.     These  are  the  organs  of 
taste  and  smell,  which  are  stimulated  chemically;  the  organs  of 
hearing  and  equilibration,  which  are  stimulated  mechanically, 
and  the  organ  of  sight,  which  is  stimulated  by  ether  vibrations. 

367.  From  direct  evidence  we  know  little  about  the  chem- 
ical senses  of  hydra,  though  as  in  the  case  of  amoeba  we  may 
infer  that  the  choice  of  food  indicates  a  sense  of  this  kind,  but 
this  evidence  is  by  no  means  conclusive.     In  the  sea  anemone, 
however,  it  is  found  by  experiment  that  the  tentacles  distin- 


156 


ANIMALS 


SENSE    ORGANS  157 

guish  food  from  other  objects  in  such  a  way  as  to  indicate  a 
chemical  sense.  No  specialized  chemical  sense  organs  have 
been  distinguished. 

368.  The  earthworm  is  chemically  sensitive  over  the  entire 
surface  of  the  body,  but  at  the  anterior  end  this  sense  is  best 
developed.     The  function  seems  to  be  located  in  sensory  cells, 
which  occur  in  clusters,  the  clusters  being  distributed  over  the 
surface  of  the  body  in  numbers  which  correspond  approxi- 
mately to  the  sensitiveness  of  the  region. 

369.  The  antennules  of  the  crayfish  are  the  seat  of  chem- 
ical  sense.     The   sensory   cells    concerned   are   not   specially 
modified,  but  their  accessory  terminal  end-organs  are  peculiar 
club-shaped  hairs,  which  are  covered  by  an  extremely  thin 
cuticula.     In  insects,  there  is  a  differentiation  of  the  chemical 
sense  into  an  olfactory  and  a  gustatory  sense.     The  former  is 
located  on  the  antennae,  and  the  sense  organ  consists  of  a  flask- 
shaped  cluster  of  sense  cells  which  are  exposed  at  the  surface 
at  the  bottom  of  a  pit  in  the  cuticula.     The  organs  of  taste 
are  similar,  but  are  found  on  surfaces  bounding  the  mouth 
cavity  or  on  the  mouth  appendages. 

370.  In  arthropods  we  first  observe  a  decided  limitation  of 
the  chemical  sense  organs  to  the  region  of  the  mouth  and  in  the 
air  breathers,  the  first  differentiation  of  the  senses  of  taste  and 
smell.     In  some  fishes,  organs  of  taste  are  found  on  the  surface 

FIG.  77. — Sense  organs.  A-E,  General  sense  organs;  F-H,  organs  of  taste; 
7,  olfactory  organ;  J-O,  eyes.  A,  General  sense  organs  of  nereis;  B,  general 
sense  organs  of  Arthropods;  C,  free  nerve  terminations  in  the  epidermis  of 
Vertebrates;  D,  sensory  corpuscles  in  the  dermis  of  Vertebrates;  E,  same,  en- 
larged; F,  diagram  of  human  tongue  showing  distribution  of  fungiform  (i)  and 
circumvallate  papillae;  G,  section  through  a  circumvallate  papilla  showing 
position  of  the  taste  buds;  H,  section  of  a  taste  bud,  showing  two  sensory  cells, 
one  supporting  cell  (c)  and  a  nerve  fibre  (g) ;  /,  section  of  the  olfactory  epithelium; 
/,  outline  of  a  small  "gliding  worm"  with  two  simple  eyes;  K,  anterior  end  of 
another  "gliding  worm"  with  a  number  of  simple  eyes  arranged  along  the  edge  of 
the  body;  L,  eye  of  a  "gliding  worm"  consisting  of  a  single  cell  with  a  sensory  brush 
partly  surrounded  by  a  pigment  cell;  M,  eye  of  a  snail  (Patella);  N,  eye  of 
nereis;  O,  part  of  N,  on  a  larger  scale,  c,  Supporting  cell;  Cu,  cuticula;  D, 
dermis;  Ep.,  epidermis;  g,  fibre  from  a  ganglion  cell;  h,  sensory  hair,  or  bristle; 
N.C.,  nerve  cell;  N.F.,  nerve  fibres;  0.  N.,  optic  nerve;  P,  pigment;  S,  sense  cells. 


ANIMALS 


of  the  body,  more  particularly  in  the  region  of  the  mouth,  but 
with  this  exception  we  can  say  that  for  all  vertebrates  the  organs 
of  taste  and  of  smell  are  limited  to  the  surface  of  the  cavities 
of  the  mouth  and  nostrils,  respectively. 

371.  The  organs  of  taste  are  called  taste  buds,  because  the 
sense  cells  are  grouped  in  small  cask-shaped  clusters.     The  taste 

buds  may  occur  in  various 
parts  of  the  mouth,  but  in 
mammals  they  are  chiefly 
found  on  the  sides  of  the 
fungiform  and  circumvallate 
papillae  (and  the  foliate 
papillae,  where  they  are 
found)  of  the  tongue.  The 
bud  consists  of  two  kinds  of 
cells,  both  very  much  elon- 
gated. One  of  these,  the 
supporting  cells,  taper  to  a 
point  at  the  free  end  while 
the  deeper  end  is  very  irregu- 
lar in  outline.  The  sensory 
cells  are  more  slender  and  end 
at  the  free  extremity  in  a  short  cuticular  hair.  At  the  other 
end  they  broaden  out  into  a  slight  enlargement.  They  have 
no  fibre  processes.  Nerve  fibres  from  deeper  lying  nerve  cells 
form  a  network  of  numerous  branches,  which  enclose  the  bud 
and  penetrate  between  the  cells  which  compose  it. 

372.  In  man  there  are  four  kinds  of  taste  sensations:  sweet, 
sour,  salt  and  bitter.     At  the  tip  of  the  tongue  sweet  is  most 
readily  detected,  sour  along  the  edges,  salt  at  the  tip  and  edges, 
and  bitter  at  the  base  of  the  tongue.     We  conclude,  therefore, 
that  these  four  sensations  are  yielded  by  as  many  different 
kinds    of    organs    which,    however,    are    not    distinguishable 
anatomically. 


FIG.  78. — Antennae  of  a  moth,  Samia 
cecropia.  A,  Of  male;  B,  of  female. 
(From  Folsom.) 


SENSE   ORGANS  159 

373.  The  sensations  yielded  by  the  olfactory  organ  are  much 
more  various  than  those  of  the  sense  of  taste,  but  at  the  present 
no   satisfactory   analysis   of   olfactory   sensations   is   possible. 
So  far  as  can  be  seen  under  the  microscope,  however,  the  sense 
organs  are  very  simple  and  all  of  the  same  kind.     They  present 
very  much  the  same  appearance  as  the  sensory  cells  of  the  epi- 
dermis of  nereis.     The  olfactory  organ  forms  a  small  part  of 
the  mucous  epithelium,  lining  the  nasal  cavity  at  its  upper 
angle.     The  epithelium  here  consists  of  columnar  cells  of  two 
kinds.     The  first  are  the  sensory  cells  which  are  very  slender 
and  end  in  a  group  of  six  to  eight  short  bristle-like  tips.     Below 
the  nucleus  the  cell  narrows  to  a  very  slender  nerve  fibre  which 
goes  to  the  brain.     Between  the  sensory  cells  are  the  somewhat 
stouter  ''supporting"  cells.     The  superficial  ends  of  these  are 
quite  regularly  prismatic  in  form,  but  below  the  nucleus  they 
are  very  irregular  in  form.     A  third  type  of  cells,  called  "  basal 
cells,"  form  the  deeper  layer  of  the  epithelium. 

THE  ORGANS  OF  SIGHT 

374.  Mention  has  already  been  made  of  the  fact  that  sensi- 
tiveness to  light  is  exhibited  by  protozoa,  which  have  no  sense 
organs.     There  are  some  protozoa,  however,  which  have  an 
"eyespot,"  a  small  speck  of  red  pigment  embedded  in  the  pro- 
toplasm.    These  eyespots  are  found  to  be  especially  sensitive 
to  light,  and  must,  therefore,  be  regarded  as  an  exceedingly 
simple  type  of  light  sense  organs. 

375.  Hydra  is  sensitive  to  light,  but  has  no  organs  specially  for 
light  perceptions.     In  some  other  Ccelenterates,  as  some  of 
the  free  swimming  medusae,  true  light-sense  organs  are  found. 

376.  Among  worms,  again,  sensitiveness  to  light  does  not 
always   indicate    the   presence    of    well-defined    sense    organs 
especially  constructed  for  this  function.     The  earthworm  is 
more  or  less  sensitive  to  light  over  the  entire  body  surface,  but 


i6o 


ANIMALS 


this  is  more  marked  at  the  anterior  end  of  the  body.  It  is 
possible  that  some  of  the  sense  organs  scattered  over  the  surface 
of  the  body  are  light-sense  organs,  but  there  is  no  direct  evi- 
dence that  such  is  the  case.  In  many  other  worms,  however, 
there  are  organs  which  are  unquestionably  eyes.  They  are 
usually  on  the  head,  but  may  be  found  else- 
where, and  they  vary  greatly  in  number. 

377.  One  of  the  simplest  of  eyes  consists  of  a 
single  epidermal  sensory  cell,  surrounded  by  a 
group  of  pigment  cells.  More  often  there  are 
a  large  number  of  sensory  cells  in  a  compact 
group.  When  this  is  the  case  the  epidermis  at 
this  point  is  greatly  thickened,  owing  to  the 
elongation  of  cells,  and  it  is  also  usually  con- 
cave toward  the  surface.  The  sensory  cells 
taper  below  the  nucleus  into  a  slender  nerve 
fibre  which  goes  to  a  deeper  lying  ganglion. 
Above  the  nucleus  the  cell  body  is  cylindrical, 
and  from  its  end  there  project  a  number  of 
slender  bristle-like  processes.  There  is  always 
considerable  pigment  in  such  an  eye.  It  lies 
either  within  the  sensory  cells  themselves  or  else 
in  the  surrounding  non-sensory  cells.  The 
cuticula  over  the  eye  is  usually  much  thick- 
ened, and  often  has  the  double  convex  form  of 
a  condensing  lens.  Often,  as  in  nereis,  the  sensory  area  sinks 
in  so  deeply  that  it  approaches  a  complete  sphere  in  form. 
It  may  then  also  separate  entirely  from  the  epidermis. 

378.  In  Arthropods  the  optic  apparatus  attains  a  much 
higher  degree  of  functional  perfection,  and  at  the  same  time 
becomes  much  more  complex.  Its  development,  however, 
proceeds  along  very  different  lines.  An  eye  similar  in  structure 
to  that  described  for  worms  is  found  in  many  Crustacea,  and 
the  ocelli  of  insects  are  also  much  the  same,  but  the  com- 


FIG.  79. — Dia- 
gram of  an  om- 
matidium.  a, 
Cuticular  cor- 
nea; b,  corneal 
cells;  c,  cone 
cells;  d,  retinal 
cells;  e,  rhab- 
dom;  /,  fibres  of 
the  retinal  cells. 


THE    EYE 


161 


pound  eyes  of  the  higher  groups  of  both  Crustacea  and  Insects 
are  of  a  different  type.  The  compound  eye  is  convex  and  it 
is  made  up  of  a  large  number  of  units  called  ommatidia.  The 
surface  of  the  cuticula  is  divided  into  numerous  polygonal  areas, 
the  "facets,"  each  of  which  corresponds  to  an  ommatidium. 
The  ommatidium  is  made  up  of  the  following  cells:  two  super- 
ficial cells  which  secrete  the  lens-shaped  cuticula;  below  these, 


FIG.  80. — Horizontal  section  through  the  right  human  eye.  a-p,  Axis  of 
vision;  ac,  central  artery;  ah,  aqueous  humor;  b,  blind  spot;  c,  conjunctiva; 
ch,  choroid  layer  of  the  eyeball;  cl,  crystalline  lens;  cmc,  circular  fibres  of  the 
ciliary  muscle;  c.m.r.,  radial  fibres  of  the  ciliary  muscle;  co,  cornea;  cp,  ciliary 
process;  cs,  canal  of  Schlemm;/0,  fovea  centralis;  on,  optic  nerve;  os,  ora  serrata, 
the  anterior  limit  of  the  sensory  portion  of  the  retinal  layer;  r,  the  retina;  sc,  the 
sclera;  sh,  sheath  of  the  optic  nerve;  vh,  vitreous  humor.  (From  Galloway). 

four  cells  which  form  an  egg-shaped  lens,  and  below  these, 
again,  seven  or  eight  sensory  cells,  so  arranged  as  to  form  a 
single  sensory  unit.  Around  the  whole  is  a  cylindrical  curtain 
of  pigment  cells.  From  the  sensory  cells,  nerve  fibres  pass 
downward  to  a  deeper  ganglion. 

379.  The  sensory  portion  of  all  invertebrate  eyes  is  developed 
from  the  epidermis,  but  the  retina  of  the  vertebrate  eye  and  all 
ii 


1 62  ANIMALS 

the  connected  nervous  elements  are  developed  from  the  brain. 
The  vertebrate  eye  is  exceedingly  complex,  and  only  the  more 
essential  features  will  be  called  to  mind:  i.  The  sclera  is  a  hol- 
low shell  of  approximately  spherical  form,  composed  of  a  thick 
and  dense  layer  of  connective  tissue.  It  is  the  protective  and 
supporting  framework  of  the  eye.  The  cornea  is  the  trans- 
parent, more  convex  portion  of  the  sclera  on  the  side  where 
the  light  enters  the  eye.  2.  The  choroid  layer  is  a  layer  of 
blood  vessels  and  capillaries,  which  lines  the  inner  surface  of 
the  sclera.  In  front  it  forms  the  iris,  and  an  opening  in  the 
latter  is  the  pupil.  3.  The  retinal  layer  is  double.  Against 
the  surface  of  the  choroid  layer  there  is  a  layer  of  pigment  cells, 
which  extends  from  the  point  where  the  optic  nerve  enters  the 
eye  to  the  pupil.  The  retina  proper  is  the  innermost  layer  and 
extends  from  the  optic  nerve  to  within  about  60°  of  the  centre 
of  the  pupil,  where  it  thins  out  into  an  endothelium,  and  as 
such,  continues  on  to  the  edge  of  the  pupil,  where  it  merges  into 
the  pigment  layer. 

380.  The  nervous  elements  of  the  retina  are  arranged  in 
three  layers:  i.  The  sensory  layer,  proper,  is  composed  of  two 
types  of  cells,  rods  and  cones,  as  they  are  called.  The  rods  are 
much  more  numerous  than  the  cones,  except  at  the  point  of 
most  distinct  vision — the  fovea  centralis — where  the  rods  are 
entirely  wanting.  The  "rods"  consist  of  a  slender  cylinder, 
which  tapers  at  one  end  into  a  short  fibre.  The  latter  is  more 
or  less  beaded  and  ends  in  a  small  knob.  The  cones  are  shorter 
and  thicker  than  the  rods  and,  as  the  name  signifies,  are  conical 
in  form.  From  the  base  of  the  cone  a  rather  stout  fibre  pro- 
ceeds, but  ends  shortly  in  a  broad  disc.  The  nuclei  of  the 
" cones"  are  rather  large  and  located  at  the  base  of  the  cone. 
The  nuclei  of  the  rods  are  smaller  and  lie  somewhere  along  the 
course  of  the  fibre.  The  cylindrical  and  conical  portions  of 
the  rods  and  cones,  respectively,  project  into  the  pigment  layer 
in  such  a  way  that  their  ends  are  completely  surrounded  by 


THE   EYE 


163 


processes  of  the  cells  of  the  pigment  layer.  The  fibre  ends  of 
the  rods  and  cones  project  toward  the  centre  of  the  eye.  2. 
The  bipolar  cells  of  the  second  layer  are  short  nerve  cells  which 
end  at  either  extremity  in  a  tuft  of  branches.  They  seem  to 
connect  the  first  and  third  layers.  3.  The  ganglion  cells  of  the 


FIG.  81. — Diagram  showing  some  of  the  retinal  elements.  Layer  i  is 
nearest  the  centre  of  the  eye  and  consists  of  nerve  fibres  (/)  which  enter  the 
optic  nerve  at  the  blind  spot;  2,  the  ganglionic-cell  layer,  made  up  of  nerve 
cells  from  which  the  fibres  (/)  arise;  3,  the  inner  molecular  layer  made  up  of 
the  minute  branches  arising  from  the  cells  of  layers  2  and  4;  4,  the  inner  nuclear 
layer,  containing  the  nuclei  of  the  short  elements  which  connect  layers  3  and  5; 
5,  the  outer  molecular  layer  which  is  similar  to  layer  3;  6,  the  outer  nuclear  layer 
— contains  the  nuclei  of  the  rod  and  cone  elements;  7,  the  layer  of  rods  (r)  and  cones 
(c);  8,  the  pigment  layer.  The  rods  and  cones  are  the  sensory  elements.  They 
project  into  the  pigment  layer.  (From  Galloway.) 

third  layer  are  large  cells  with  a  bush  of  protoplasmic  processes 
and  a  long  fibre.  The  fibres  form  a  layer  on  the  surface  of  the 
retina  next  the  centre  of  the  eye.  They  all  converge  to  the 
point  where  the  optic  nerve  enters  the  eye.  It  is  these  fibres 
with  their  medullary  sheaths  that  constitute  the  optic  nerve. 


164 


ANIMALS 


381.  Just  inside  the  pupil  lies  a  double  convex  lens.     It 
originates  from  the  epidermis  by  an  infolding.     It  is  the  densest 
organic  structure  of  the  body.     It  is  fibrous  in  structure  and  of 
glassy  transparency.     The  lens  is  enclosed  in  a  capsule,  which 
is  attached  by  means  of  fibres  to  the  muscular  ciliary  body,  a 
portion  of  the  choroid  layer. 

382.  The  large  central  cavity  of  the  eye  is  filled  with  a  trans- 
parent jelly,  the  vitreous  humor.     The  smaller  space  in  front 
of  the  iris  contains  a  more  fluid,  aqueous  humor. 


FIG.  82. — Diagrams  to  show  how  the  concave  and  convex  arrangement  of 
the  sensory  elements  in  invertebrate  eyes  serves  to  indicate  the  direction  of 
the  light  rays.  A,  The  concave  eye,  like  that  of  Patella;  B,  the  convex  eye, 
like  that  of  the  compound  eyes  of  Arthropods. 

383.  Vision. — The  simplest  type  of  eye  described  doubtless 
enables  the  possessor  to  distinguish  more  readily  differences  in 
the  intensity  of  light,  such  as  a  passing  shadow.  The  animal 
would  not  be  able,  however,  to  distinguish  one  object  from 
another  by  its  form,  since  the  eye  is  not  so  constructed  as  to 
form  an  image.  Where  there  are  a  number  of  such  eyes  so 
placed  on  the  body  that  they  "look"  in  different  directions,  the 
stimulation  of  one  more  than  another  would  be  an  indication 
of  the  direction  of  the  source  of  light.  In  the  slightly  more 
complex  eye,  where  the  elements  are  arranged  radially  on  a 
concave  surface,  there  is  formed  a  crude  image,  because  the 


VISION  165 

arrangement  of  the  pigment  and  the  sensory  elements  deter- 
mines that  each  element  is  stimulated  from  a  particular  direc- 
tion. The  image  in  this  case  is  formed  by  projection.  If 
there  is  a  cuticular  lens  it  is  too  close  to  the  retina  to  form  an 
image;  it  only  serves  to  concentrate  the  light. 

384.  When  the  sensory  elements  are  arranged  on  a  convex 
surface  as  in  the  insect  eye,  an  image  is  also  formed  by  pro- 
jection, but  in  this  case  the  image  is  erect  while  in  the  concave 
eye  it  is  inverted.     However,  the  cuticular  lens  and  the  cone 
of  the  ommatidium  are  so  placed  that  an  image  is  formed  in 
the  plane  of  the  retinal  cells.     There  is,  therefore,  a  combina- 
tion of  image  by  projection  and  image  by  refraction.     This 
type  of  eye  is  comparatively  efficient.     Form  is  distinguished 
with  considerable  detail,  and  colors  are  recognized,  but  there 
is  still  a  deficiency  which  makes  the  insect  eye  decidedly  in- 
ferior to  the  vertebrate  eye.     There  is  no  provision  for  focusing. 
It  is  possible  that  the  great  depth  of  the  sensory  element  (the 
rhabdom)  is  in  some  measure  a  compensation;  the  multiplicity 
of  eyes  is  another.     In  some  cases  a  single  eye  is  so  constructed 
that  one  part  is  adapted  for  far  vision,  the  other  for  objects 
near  at  hand. 

385.  In  some  Vertebrates  (fishes)  the  eye  is  focused  by  mov- 
ing the  lens  toward  or  away  from  the  retina.     A  more  refined 
method  is  adopted  by  the  higher  Vertebrates.     The  lens  is 
elastic  and  is  continually  flattened  somewhat  by  the  pressure 
of  the  capsule  on  the  anterior  and  posterior  surfaces  of  the  lens. 
By  the  contraction  of  the  ciliary  muscles  this  pressure  is  some- 
what removed  and  the  lens,  by  elasticity,  assumes  a  more  con- 
vex form. 

386.  In  many  eyes  the  quantity  of  light  admitted  to  the 
sensory  elements  is  controlled  by  movements  in  the  pigment 
cells.     When  the  light  is  too  intense  the  pigment  advances 
and  cuts  off  some  rays.     In  weak  light  the  pigment  recedes, 
thus  admitting  a  broader  beam  of  light.     This  adjustment  is 


1 66  ANIMALS 

well  developed  in  the  insect  eye.  In  the  vertebrate  eye  this 
adjustment  is  supplemented  by  a  change  in  the  size  of  the  pupil. 
A  circular  muscle  in  the  iris  causes  the  pupil  to  contract  while 
a  set  of  radial  muscles  cause  it  to  expand. 

HEARING  AND  EQUILIBRATION 

387.  Statocysts. — Many  of  the  lower  animals  have  been 
credited  with  a  sense  of  hearing,  but  it  is  very  doubtful  whether 
any  aquatic  invertebrate  has  really  an  organ  for  perceiving 
sound.  That  many  aquatic  animals  may  "feel"  and  respond 
to  vibrations  set  up  in  the  water  is  quite  probable.  But  this 

may  be  due  to  the  stimulation  of 
other  organs,  such  as  the  tactile 
sense  organs.  The  organs  found 
in  jellyfishes,  worms,  Crustacea, 
and  many  other  aquatic  inverte- 
brates, which  have  been  called 
"ear  sacs,"  are  well  understood 
and  are  more  properly  called 
statocysts. 

FIG.  83.-Statocyst  of  a  Mol-  388.    In    hydra    there   are   no 

hisc.  »,  Nerve;  o,  otolith;  s.c.,  statocysts,  nor  are  they  found  in 
sensory  cells.  (From  Galloway, 

after  ciaus.)  any  other  fixed  forms.      In   the 

hydromedusae,  however,  they  are 

very  common.  They  consist,  typically,  of  a  deep  sack-like 
depression  of  the  ectoderm,  which  contains  sensory  cells  and 
a  statolith.  The  sack  may  be  open  or  closed,  but  in  either 
case  is  filled  with  a  fluid.  The  sensory  cells  are  provided 
with  bristle-like  processes  which  project  into  the  cavity  of 
the  statocyst.  The  statoliths  are  heavy  concretions  of  inor- 
ganic matter  which  stimulate  the  sensory  cells  by  contact 
with  the  bristles.  When  the  animal  turns  over  in  swimming, 
the  statoliths,  by  their  weight,  always  settle  to  the  lower  side 


STATOCYSTS  167 

of  the  statocyst  and  stimulate  the  cells  in  that  region.  By 
this  means  the  organism  is  informed  of  the  orientation  of  its 
body  in  space. 

389.  No  statocysts  are  found  in  either  nereis  or  the  earth- 
worm, but  they  are  present  in  some  other  Annelids. 

390.  On  the  upper  surface  of  the  basal  portion  of  the  anten- 
nules  of  the  crayfish  there  is  a  small  opening  which  leads  into  a 
statocyst.     The  inside  of  the  sack  is  lined  with  sensory  hairs, 
upon  which  rests  the  statolith.     In  this  case  the  statolith  is 
composed  of  grains  of  sand  cemented  together  by  a  secretion 


FIG.  84. — Diagram  of  the  internal  FIG.  840,. — Diagram  of  the  laby- 

ear  (labyrinth)  of  one  of  the  lower        rinth   of   a   mammal    showing  the 
vertebrates,    u,  Utriculus  with  three         cochlea, 
semicircular  canals;  s,  sacculus;  /, 
lagena. 

of  the  epidermis.  The  sand  is  introduced  into  the  sack  by  the 
animal  itself  after  each  ecdysis,  for  the  lining  of  the  sack  " sheds" 
like  the  remainder  of  the  cuticula,  and  its  contents  are  cast 
out  at  the  same  time. 

391.  Statocysts  are  practically  wanting  in  insects. 

392.  The  Vertebrate  Organ  of  Equilibration. — The  internal 
ear  of  vertebrates  consists  of  a  membranous  sack,  the  labyrinth, 
which  is  lined  internally  with  a  layer  of  cells  of  ectodermal  origin. 
At  certain  places  in  this  lining  there  are  groups  of  sensory  cells, 
which  have  a  close  resemblance  to  the  sensory  cells  of  the  stato- 
cysts just  described.     The  labyrinth  is  filled  with  a  fluid  and 


1 68  ANIMALS 

contains  a  large  calcareous  concretion,   the  "ear  stone,"  or 
numerous  smaller  particles  which  are  called  ear  sand. 

393.  The  labyrinth  of   the  round-mouth   eels  is   a  simple 
ovoidal  sack,  but  in  the  higher  fishes  the  sack  is  partly  divided 
into  two  chambers,  a  utriculus  and  a  saculus,  and  connecting 
with  the  utriculus  are  three  semi-circular  canals,  two  of  which 
are  in  vertical  planes  but  at  right  angles  to  each  other,  while 
the  third  canal  is  horizontal. 

394.  In  the  higher  vertebrates  the  utriculus  with  the  three 
semi-circular  canals,  and  the  sacculus  are  also  found,  and,  in 
essential  features,  the  same  as  in  the  higher  fishes. 

395.  The  function  of  this  part  of  the  vertebrate  ear  is  the 
same  as  that  served  by  the  statocyst  of  the  invertebrates.     It 
has  nothing  to  do  with  hearing.     It  is  an  organ  of  orientation 
and  equilibration.     If  the  organ  is  destroyed   or   the   nerve 
leading  to  it  severed,  the  animal  has  difficulty  in  maintaining 
its  normal  upright  position.     A  fish,  for  example,  which  has 
lost  the  use  of  this  organ  no  longer  swims  in  its  normal  way.     It 
turns  over  and  over,  or  may  swim  with  its  back  downward. 

396.  The  Auditory  Organ. — The  sense  of  hearing  seems  to  be 
primarily  developed  in  connection  with  voice,  and  it  is  doubtful 
whether  there  is  any  species  in  which  one  occurs  without  the 
other.     Within  the  class  Insecta  we  find  the  only  invertebrates 
having  sense  organs  for  the  perception  of  sound,  and  the  species 
in  which  they  occur  best  developed  are  our  singing  insects,  the 
grasshopper,  katydid,  cricket,  and  cicada  (" locust,"  "  harvest 
fly,"  "jar  fly").     The  singing  in  these  cases  is  usually  done  by 
the  male,  and  is  intended  for  the  "ears"  of  the  female.     Many 
sounds  are  produced  by  animals,  which  are  accidental,  and  can- 
not be  called  voice,  as  in  most  cases  the  buzzing  produced  by 
the  wings  in  flight.     At  the  same  time  the  buzz  of  the  wings 
may,  in  some  cases,  be  used  as  a  means  of  communication  be- 
tween individuals,  in  which  case  it  would  have  to  be  regarded 
as  voice.     On  the  other  hand,  sound  vibrations  may  be  per- 


THE   EAR  169 

ceived  by  sense  organs  other  than  that  of  hearing,  hence  a 
response  to  a  sound  is  not  necessarily  an  indication  of  the 
presence  of  an  organ  of  hearing.  It  must  also  be  kept  in  mind 
that  other  animals  may  make  and  hear  sound  vibrations  of  so 
high  a  pitch  as  to  be  inaudible  to  the  human  ear. 

397.  The   ear  of   the  grasshopper  is   called   a   tympanum, 
because  of  its  resemblance  to  a  drum  head.     It  is,  in  fact,  a 
thin  membrane  stretched  over  a  large  respiratory  cavity,  and 
is  located  on  the  side  of  the  first  abdominal  segment.     The 
katydid  and  cricket  also  have  tympanums,  but  they  are  located 
on  the  tibia  of  the  anterior  legs.     But  in  the  essential  points 
these  organs  are  similar  in  structure.     The  sensory  apparatus 
consists  of  groups  of  sensory  cells,  intimately  connected  with 
the  inner  surface  of  the  tympanum.     The  tympanum  is  highly 
responsive  to  vibrations  of  the  air,  and  by  its  own  vibrations 
the  connected  sensory  cells  are  stimulated. 

398.  At  one  side  of  the  sacculus,  in  frogs  and  reptiles,  there 
is  a  small  pocket  which  is  not  found  in  fishes.     In  birds  this 
pocket  becomes  a  long  tube,  and  in  mammals  it  is  very  long 
and  coiled.     This  is  the  cochlea  and  is  the  true  organ  of  hearing. 
On  one  side  of  the  cochlea  the  lining  epithelium  is  composed  of 
peculiarly  arranged  columnar  cells,  which  form  what  is  known 
as  the  organ  of  Corti.     The  cochlea  is  filled  with  a  fluid,  endo- 
lymph,  like  the  other  parts  of  the  labyrinth.     In  a  cross  section 
of  the  organ  of  Corti  there  are  several  supporting  cells  and  about 
four  sensory  cells,  but  in  a  longitudinal  section  there  would  be 
from  4,000  to  5,000  sensory  cells,  covering  a  space  of  more  than 
25  mm.     The  sensory  cells  are  rather  stout  and  rounded  at 
the  lower  end.     At  the  free  end  they  each  bear  about  twenty 
rod-like  processes,  which  project  into  the  endolymph.     This 
organ  rests  on  a  membrane  of  fibres  (the  basilar  membrane) 
which  stretches  across  from  the  bony  wall  of  one  side  to  that  of 
the  other.     Above  the  sensory  cells,  suspended  in  the  endo- 
lymph, is  a  thick  cuticular  membrane  (membrana  tectoria), 


170  ANIMALS 

which  almost  touches  the  processes  of  the  sensory  cells.  This 
membrane  is  free  at  one  edge,  but  attached  at  the  other  to  a 
non-sensory  portion  of  the  cochlea.  The  nerve  fibres  supplying 
this  organ  end  in  free  nerve  terminations  around  the  sensory 
cells.  The  basilar  membrane  becomes  wider  toward  the  apex 
of  the  cochlea,  and  the  fibres  of  the  basilar  membrane  become 
correspondingly  longer. 

399.  The  entire  labyrinth  lies  in  a  cavity  of  approximately 
the  same  shape,  in  the  petrosal  portion  of  the  temporal  bone. 
This  is  the  bony  labyrinth.     It  is  considerably  larger  than  the 
membranous  labyrinth,  and  the  space  between  is  filled  with 
perilymph.     A  small  opening  in  the  wall  of  the  bony  labyrinth 
is  covered  by  a  membrane,  and  a  small,  movable  bone,  the 
stapes.     By  the  vibrations  of  these  parts  the  perilymph  is 
disturbed  and  through  it  the  fibres  of  the  basilar  membrane  and 
thus  the  cells  in  the  organ  of  Corti  are  stimulated.     It  is 
supposed  that  the  difference  in  the  lengths  of  the  basilar  mem- 
brane fibres  corresponds  to  differences  in  the  lengths  of  sound 
waves,  and  that,  therefore,  sounds  of  a  given  pitch  stimulate 
only  that  part  of  the  organ  of  Corti  in  which  fibres  of  a  cor- 
responding length  occur. 

400.  Certain  accessory  organs,  by  which  the  sound  waves 
in  the  air  are  transmitted  to  the  fluids  of  the  labyrinth  are 
found  in  all  animals  having  a  well-developed  sense  of  hearing, 
and  the  condition  of  these  organs  is  a  very  good  index  as  to  the 
degree  of  perfection  of  the  sense  itself. 

401.  The  auricle,  or  shell,  of  the  outer  ear  is  found  only  in 
Mammals.     Its  function  is  manifestly  to  gather  the  sound 
waves  and  direct  them  to  the  auditory  meatus,  the  tube  which 
leads  to  the  ear  drum.     The  external  auditory  meatus  is  also 
found  in  Birds  and  some  Reptiles.     But  in  the  frogs  the  ear 
drum  is  on  a  level  with  the  surface  of  the  head.     Some  Reptiles 
(snakes),    some    Amphibia    (salamanders)    and    Fishes    have 
neither  outer  nor  middle  ear. 


THE    EAR 


171 


402.  The  ear  drum  is  a  tightly  stretched  membrane,  which 
is  so  constructed  that  it  vibrates  equally  well  with  sounds  of 
different  pitch. 

403.  Between  the  ear  drum  and  the  inner  ear  there  is  a  small 
cavity  which  communicates  with  the  pharynx  through  the 


FIG.  85.  FIG.  87. 

FIG.  85. — Cross  section  of  one  turn  of  the  cochlear  spiral  as  it  lies  in  position 
in  the  long  labyrinth.  The  organ  of  Corti  (above  the  letter  C)  rests  on  the 
basilar  membrane  and  nerve  fibres  run  out  to  the  spiral  ganglion  N. 

FIG.  86. — Part  of  the  organ  of  Corti,  to  show  the  sensory  cells  and  the  nerve 
fibres  leading  to  the  spiral  ganglion. 

FIG.  87. — Diagram  of  the  middle  ear  of  a  mammal.  E,  External  auditory 
passage,  ending  at  the  ear  drum;  /,  internal  ear;  M ,  middle  ear,  opening  into  the 
pharynx  by  the  Eustachian  tube  E.T.;  i,  malleus;  2,  incus;  3,  stapes,  fitting 
into  the  oval  window. 

Eustachian  tube.  This  cavity  is  the  middle  ear.  Its  most 
important  parts  are  three  small  bones,  the  hammer,  the  anvil 
and  the  stirrup,  through  which  the  vibrations  of  the  ear  drum 
are  transmitted  to  the  perilymph.  The  hammer  is  attached 
to  the  ear  drum,  the  stapes  fits  over  the  opening  in  the  bony 
labyrinth,  and  the  two  are  connected  by  the  anvil.  In  Birds 


172  ANIMALS 

and  those  Reptiles  and  Amphibia  which  have  an  ear  drum  the 
bones  of  the  middle  ear  consist  of  one  piece  only,  the  columella. 

404.  Caution. — Our  knowledge  of  the  senses  of  animals  is  still 
far  from  complete.     Since  we  cannot  experience  the  sensations 
of  another  human  being  we  can  only,  in  a  general  way,  infer  what 
they  are  by  supposing  them  to  resemble  our  own.     Such  an 
assumption,  with  regard  to  the  lower  animals,  is  of  little  value. 
We  have  senses  of  which  we  are  unconscious  (the  sense  of  the 
organ  of  equilibration  and  the  senses  of  the  deep  lying  organs 
mentioned  above),  and  the  lower  animals  may  have  senses 
which  we  do  not  have.     The  lateral  line  of  Fishes,  for  example, 
is  a  system  of  sense  organs  by  which  the  animal  is  informed  of 
movements  of  the  water.     Many  other  sense  organs  have  been 
found  whose  function  is  unknown. 

405.  Function  of  the  Senses. — Concerning  sense  organs  in 
general,  it  may  be  said  that  animals  are  provided  with  sense 
organs  for  perceiving  those  changes  in  the  environment  which 
might  operate  either  to  the  advantage  or  detriment  of  the  or- 
ganism, and  to  which  the  organism  is  capable  of  making  an  ef- 
fective response.     Within  the  meaning  of  the  term  as  used  here, 
we  are  insensible  to  those  constant  elements-  of  the  normal  at- 
mosphere,   oxygen,    carbon-dioxide  .and    nitrogen,    although 
oxygen  is  absolutely  and  constantly  necessary  to  life,  while 
carbon-dioxide  in  large  quantities  is  fatal.     Nor  do  we  possess 
organs  for  dectecting  changes  in  the  force  of  gravity,  of  at- 
mospheric pressure  or  of  electrical  conditions,  for  the  evident 
reason  that  either  the  welfare  of  the  organism  is  not  affected 
by  the  changes  which  normally  occur  or  else  that  no  effective 
response  is  possible. 

ORGANS  OF  RESPONSE 

406.  When  an  animal  is  sufficiently  stimulated  a  response 
occurs.     This  is  usually  in  the  form  of  a  contraction  or  expan- 


ORGANS   OF   RESPONSE  173 

sion  or  a  combination  of  both.  Very  often,  however,  response 
takes  the  form  of  glandular  activity.  Some  times  light  is 
produced  and  some  times  an  electrical  discharge.  The  latter 
responses  are  relatively  rare.  We  will  now 
consider  the  organs  of  response;  and  first  the 
muscles. 

407.  When  an  expanded  amoeba  is  strongly 
stimulated  it  contracts  into  a  spherical  mass. 
How  this  is  done  we  do  not  know.      It  is  a 
property    of    undifferentiated    protoplasm    in 
which  no  contractile  elements  of  any  kind  can 
be  distinguished.       In    some    other    protozoa 
(paramcecium  and  stentor)   there  are  distinct 
contractile    elements   in   the   form   of   slender 
fibrils    (myonemes),   which  traverse  the   ecto- 
plasm in  a  longitudinal,   slightly  spiral   direc- 
tion.    By  their  contraction  they  also  cause  the 
animal  to  assume  a  more  nearly  spherical  form. 
Such    cells   contract   more   energetically   than 
does  the  amoeba. 

408.  In  hydra,  the  contractile  fibrils  of  each 
cell   are   grouped   into    a  bundle,  the  muscle 
fibre,  which  is  much  longer  than  the  body  of 
the  cell  and  projects  on  either  side.     This  gives 
the  cell  the  form  of  a  T  with  a  very  short  stem, 
representing  the  cell-body  and  the  cross  bar 
representing  the  fibre.     The  fibres  of  the  ecto- 
derm cells  run  longitudinally,  while  those  of 
the  entoderm  run  circularly  around  the  body. 
In  this  case  part  of  the  cell  remains  undiffer- 
entiated  and   continues    to  form  part  of  an 
epithelium.     This  condition  is  also  met  with  in  others  of  the 
lower  phyla,  but  in  the  Annelids  and  higher  forms  the  differentia- 
tion proceeds  farther  and  involves  the  entire  cell.     So  we  find 


174 


ANIMALS 


that  besides  the  epithelial  layers  which  cover  the  exterior  of  the 
animal  and  line  the  internal  cavities,  there  are  also  other  tissues 
like  the  muscles  which  lie  between  the  ectoderm  and  entoderm. 
These  other  tissues  constitute  a  third  layer,  which  is  called 


Ect. 


FIG.  89. — Structure  of  the  body-wall  of  hydra.  A,  Part  of  a  cross  section 
of  the  column;  B,  the  region  between  ectoderm  and  entoderm,  more  highly 
magnified  to  show  the  longitudinal  and  circular  muscle  fibres;  C,  diagram  of 
an  ectoderm  cell  with  a  muscle-fibre  process.  Ect,  ectoderm;  En,  entoderm; 
/,  muscle  fibre;  N,  nematocysts;  s,  supporting  layer  between  ectoderm  and 
entoderm. 

mesoderm  (in  the  embryonic  stage  of  development) .  This  layer 
is  entirely  wanting  in  hydra  and  is  not  well  developed  in  any 
of  the  Ccelenterates. 

409.  The  muscles  of  the  Annelids  are  composed  of  fibres, 
and  each  fibre  consists  of  a  bundle  of  muscle  fibrils.  Each 


FIG.  90. — A  branched  muscle  fibre  from  the  wall  of  a  blood-vessel  (Nereis). 

fibre  has  a  nucleus  and  represents  an  elongated  cell.  The  fibres 
taper  to  a  point  at  each  end  and  are  arranged  parallel  in 
masses  called  muscles.  In  the  worm  the  principal  muscles  are 
arranged  in  two  sets.  Just  beneath  the  epidermis  there  is  a 
thin  layer  which  runs  circularly  around  the  body,  and  beneath 


MUSCLES 


175 


this  there  are  four  large  masses  which  run  longitudinally,  two 

on  the  dorsal  side  and  two  on  the  ventral. 
410.  The  muscle  systems  and  the  integument  form  a  hollow 

cylinder,  which  is  closed  at  both  ends  and  filled  with  a  fluid, 

the  body  fluid.  This  constitutes  a  locomotor  mechanism, 
which  operates  as  follows:  When  the 
circular  muscles  contract  the  body  be- 
comes more  slender,  and  must,  there- 
fore, elongate,  which  causes  the  longi- 
tudinal muscles  to  expand.  When  the 
longitudinal  muscles  contract  the  body 
is  shortened  and  must  become  corre- 
spondingly thicker,  which  causes  the 
circular  muscles  to  expand.  The  ac- 
tivity of  a  muscle  is  always  expressed 
by  contraction.  When  it  expands  it 
is  passive,  the  action  being  due  to  the 
contraction  of  other  muscles.  The 
worm  is  provided  witri  groups  of 


FIG.  91. — Plain  muscle 
fibres.  n,  Nucleus;  p. 
protoplasm;  p',  muscle 
fibrils. 


FIG.  92.— Three  muscle  cells  of  nereis  in  cross  sec- 
tion. The  dots  on  the  periphery  of  the  cell  are  the 
muscle  fibrils  in  cross  section. 


bristles  (setae)  on  either  side  of  each  segment.  These  bristles 
can  be  set  forward  or  backward  by  means  of  small  ifluscles 
connected  with  them.  The  setae  prevent  slipping  ofthe  body 
in  the  direction  in  which  they  are  set.  If  now  the  setae  are  set 
backward  and  the  circular  muscles  contract,  the  anterior  end  of 
the  animal  moves  forward,  the  posterior  end  remaining  fixed. 
Then,  when  the  longitudinal  muscles  contract,  the  posterior 
end  moves  forward,  and  the  anterior  end  remains  fixed.  A 


176  ANIMALS 

I 

repetition  of  these  events  causes  another  hitch  forward,  and  so 
the  worm  progresses  by  a  process  called  inching. 

411.  If  the  setae  are  set  forward  and  the  muscular  contraction 
repeated  in  the  same  sequence  as  before,  the  animal  inches  back- 
ward, and  practically  with  equal  facility.     This  is  the  chief 
method  of  locomotion  of  the  earthworm,  but  nereis,  by  wave- 
like  contractions  of  the  longitudinal  muscles,  alternately  right 
and  left,  causes  a  serpentine  movement  of  the  body  by  which 
it  creeps  along.     A  similar  mode  of  contraction,  alternating 
between  the  dorsal  and  ventral  muscles,  causes  an  undulatory 
up  and  down  motion  by  which  nereis  swims.     The  leech  swims 
in  the  same  way. 

412.  In  Arthropods,  the  method  of  locomotion  is  totally 
different.     Here  the  animal,  both  body  and  appendages,  is 
encased  in  a  series  of  rings  or  cylinders  composed  of  the  stiff 
cuticula  and  permitting  of  practically  no  change  of  form  within 
the  individual  segments.     However,  the  segments  are  flexibly 
connected  with  each  other  and  provided  with  muscles  attached 
in  such  a  way  that  the  segments  may  be  moved  with  respect 
to  each  other.     Thus  the  ambulatory  appendage  of  the  cray- 
fish consists  of  six  segments,  connected  with  each  other  and  with 
the  body  by  six  hinge-like  joints.     The  muscles  lie  inside  the 
cylindrical  segments  of  integument  to  which  they  are  attached 
and  extend  across  the  joint  from  segment  to  segment.     When 
a  muscle  contracts,  it  flexes  the  appendage  at  the  joint  between 
the  two  points  of  attachment.     The  muscles  are  arranged  in 
pairs  at  each  joint,  and  the  two  muscles  of  a  pair  move  the  ap- 
pendage   in    opposite    directions.     The    appendages    operate 
purely  as  levers,  and  locomotion  is  wholly  due  to  leverage  action. 
This  is  true  also  of  the  action  of  the  tail  fin,  which  by  its  power- 
ful strokes   causes  the  body  to  shoot  backward.     The  wings 
of  insects  likewise  operate  as  levers.     In  these  cases  the  ful- 
crum of  the  leverage  action  is  the  water  or  air  instead  of  the 
earth. 


MUSCLES  177 

413.  Locomotion  in  Vertebrates  is  similar  to  that  of  Arthro- 
pods in  principle,  with  this  important  difference:  The  levers 
occupy  the  axis  of  the  appendage  while  the  muscles  are  attached 
to  the  surface  and  lie  outside.     A  mechanical  advantage  is  here 
obtained  by  the  greater  flexibility  of  the  joints.     The  hinge 
joint,  which  is  the  only  one  possible  in  Arthropods,  permits  of 
motion  in  one  plane  only,  while  the  ball  and  socket  joint,  which 
is  found  at  many  points  in  the  vertebrate  skeleton,  gives  uni- 
versal motion. 

414.  The  muscular  tissue  of  the  body- wall  and  the  organs  of 
locomotion,   is   composed   of   fibres   of   a   complex   structure. 
Almost  the  entire  substance  of  the  cell  is  transformed  into 
muscle  fibrils,  of  which  there  are  a  large  number.     There  is 
also  a  fibre  sheath  which  binds  the  fibrils  together,  and  among 
the  fibrils  are  a  number  of  nuclei.     This  type  of  fibre  is,  there- 
fore, not  a  uninuclear  cell.     The  most  striking  characteristic  of 
these  fibres  is  the  banded  appearance  which  they  present  under 
the  microscope.     The  light  is  affected  differently  at  different 
points  in  the  fibre  so  that  some  appear  light  and  others  dark. 
These  points  alternate  regularly  and  give  the  fibre  the  appear- 
ance of  being  crossed  by  alternating  dark  and  light  bands.     Such 
muscular  tissue  is  called  cross  striped  or  striate  and  distinguishes 
the  skeletal  muscles  of  Vertebrates  and  Arthropods  from   the 
muscles  of  Worms  and  most  other  invertebrates.     The  heart  of 
Vertebrates  is  also  composed  of  striate  muscle,  but  the  muscles 
of  the  digestive  tract  and  many  other  parts  of  the  body  are  more 
like  those  of  the  Annelids.     They  are  called  smooth  muscle 
fibres.     Generally  the  cross-striped  muscles  are  more  quick  and 
vigorous  in  action  than  the  smooth  muscle  fibres.     See  Fig  101. 

SKELETON  AND  CONNECTIVE  TISSUE 

415.  Between  the  ectoderm  and  entoderm  of  hydra  there  is 
a  thin  layer  called  the  supporting  lamella.     It  is  secreted  by  the 
cells  of  the  ectoderm  and  entoderm,  and  is,  therefore,  not  a 

12 


1 78  ANIMALS 

cellular  layer.  It  is  of  a  firm  gelatinous  substance,  which 
probably  adds  to  the  rigidity  of  the  body.  In  other  Ccelenter- 
ates  this  layer  is  much  thicker,  and  especially  in  jelly  fishes  it 
forms  the  major  portion  of  the  mass  of  the  animal.  In  these 
cases  it  may  contain  cellular  elements  of  various  kinds,  which 
have  moved  into  it  from  the  ectoderm  and  entoderm. 

416.  Worms  generally  have  no  skeleton,  but  special  tissues 
are  sometimes  developed  in  connection  with  certain  organs. 
The  nervous  system  of  nereis  is  enclosed  by  a  thick  covering 
of  a  non-cellular  connective  tissue. 

417.  The  chitinous    cuticula    of    the   Arthropods    forms    a 
highly  efficient  exoskeleton.     To  this  the  muscles  and  other 
internal  organs  are  attached  through  the  medium  of  the  epi- 
dermis and  basement  membrane.     The  latter  is  a  thin  non- 
cellular  layer  secreted  by  the  epidermis  on  its  inner  surface. 

418.  In  the  Mollusca,  also,  the  exoskeleton  is  usually  ex- 
tremely well  developed.     In  this  case  it  consists  chiefly  of  thick 
layers  of  limey  salts,  deposited  between  the  cuticula  and  the 
epidermis.     This  " shell"  is  formed  only  by  the  epidermis  of  a 
special  fold  of  the  skin  called  the  mantle. 

419.  An  exoskeleton  is  rather  exceptional  among  Vertebrates. 
The  scaly  covering  of  the  body  of  most  Fishes  is  an  excellent 
protective  structure,  but  is  rather  too  flexible  to  be  called  an 
exoskeleton.     In  the  head  region,  however,  the  scales  are  often 
so  intimately  united  as  to  form  a  real  supporting  shell.     In 
special  cases  this  shell  is  extended  over  a  large  portion  of  the 
body,  as  in  the  gar  pike  and  trunk  fishes. 

420.  The  shell  of  the  turtles  is  composed  partly  of  the  ex- 
panded ribs  of  the  endoskeleton  and  partly  of  bony  plates  formed 
in  the  skin.     The  whole  of  the  bony  portion  is  covered  by  horny 
plates  developed  by  the  epidermis. 

421.  The  jointed  shell  of  the  armadillo  is  made  up  of  bands  of 
dermal  bone,  while  the  covering  of  the  scaly  ant-eater  is  com- 
posed of  large,  horny  scales. 


THE    SKELETON  179 

THE  ENDOSKELETON 

422.  A  well-developed  endoskeleton  is  found  only  in 
Vertebrates,  but  the  notochord  of  the  lancelet  may  be  regarded 
as  an  endoskeleton  of  the  simplest  form.  It  is  simply  a  rod  of 
large  turgid  cells  with  strong  cell  membranes.  This  rod  ex- 
tends lengthwise  of  the  body,  immediately  under  the  spinal 
cord.  It  is  also  found  well  developed  and  functional  in  the 
round-mouth  eels,  but  in  other  fishes  the  vertebrae  are 


FIG.  93. — Outline  drawing  of  the  lancelet  (Branchiostoma)  to  show  the  position 
of  the  notochord  (N.C.)  and  the  spinal  cord  (S.C.). 

formed  around  it  and  take  its  place,  though  traces  of  it  remain 
in  the  adult.  In  the  higher  Vertebrates  it  is  formed  in  the 
embryo,  but  all  evidence  of  it  disappears  with  the  development 
of  the  spinal  column. 

423.  The  first  evidence  of  a  true  internal  skeleton  occurs 
in  the  round-mouth  eels.     Here  small  pieces  of  cartilage  are 
formed  around  the  notochord  and  spinal  cord.     These  pieces 
do  not  unite  to  form  vertebrae,  but  they  are  arranged  in  a  series 
segmentally.     Beneath  the  brain  and  around  the  pharynx  a 
large  number  of  similar  cartilages  occur.  In  the  sharks  and  rays 
and  some  other  fishes  like  the  sturgeon,  the  skeleton  is  also 
cartilaginous,  but  better  developed.     There  is  a  continuous 
column  of  vertebrae  and  a  skull. 

424.  In  the  higher  Fishes  and  all  the  higher  classes  of  Verte- 
brates the  skeleton  is  also,  at  first,  cartilage,  but  this  is  gradually 
transformed  into  true  bone.     Certain  parts  of  the  skeleton 
remain  cartilaginous  throughout  life,  even  in  the  highest  forms. 

425.  Cartilage  is  composed  of  cells  which  secrete  an  ex- 
tremely firm  gelatinous  substance.     This  substance  is  secreted 


i8o 


ANIMALS 


in  such  large  quantities  that  the  cells  themselves  come  to  lie 
far  apart  in  the  jelly.  Cartilage  is  sometimes  semi-transparent. 
Sometimes  it  contains  fibres,  and  very  often  it  is  hardened 
by  deposits  of  lime  salts. 

426.  When  true  bone  is  formed,  it  either  takes  the  place  of 
cartilage  or  else  is  formed  where  no  cartilage  had  previously 
existed.     In  the  first  case,  the  cartilage  is  first  dissolved  and 
in  its  place  solid  masses  of  lime  salts  are  laid  down,  layer  upon 

layer,  by  special  bone-forming 
cells.  Some  cells  become  em- 
bedded in  the  bone  and  these 
are  connected  with  each  other 
by  slender  protoplasmic  threads. 
The  cells  and  their  connecting 
threads  form  the  lacunae  and 
canaliculae  of  the  dry  bone. 
Around  the  blood  vessels  the 
FIG.  94.— Cartilage  cells  lying  singly,  bone  is  deposited  in  concentric 

or  in  small  groups  of  two  or  three,  in  lovpro  K1lt  PlcpWV,prP  tV»p  lavpr* 
the  cartilage  jelly  which  is  secreted  by  iayers>  c  ut  eisewn<  3  lay 61 

them.  are  parallel  with  the  surface  of 

the  bone.     The  layers  are  called 

lamellae;  the  spaces  occupied  by  the  blood  vessels,  Haversian 
canals. 

427.  When  the  bone  does  not  take  the  place  of  a  cartilage, 
it  is  formed  in  connective  tissue.     Such  bone  is  called  mem- 
brane bone. 

428.  The  skeleton  is  bound  together  by  bands  of  exceedingly 
strong,  elastic  connective  tissue  called  ligaments.     They  are 
found  at  the  joints,  binding  bone  to  bone,  so  as  to  keep  each  in 
its  place.     They  are  not  connected  with  the  muscles. 

429.  The  muscles  are  sometimes  connected  directly  with  the 
bones,  sometimes  indirectly  through  the  medium  of  tendons, 
which  are  bands  of  inelastic  connective  tissue.     The  muscles 
which  produce  motion  at  a  given  joint  must  be   connected 


THE    SKELETON 


181 


across  the  joint  from  bone  to  bone.  But  a  mass  of  muscle 
around  a  joint  would  impede  motion.  The  tendon  of  a  muscle 
is  not  nearly  so  thick  as  the  muscle,  consequently  where  free- 
dom of  motion  is  important,  the  muscle  is  frequently  connected 


FIG.  95. — Bone,  in  cross  section.  In  A  the  surface  of  the  bone  is  uppermost; 
B,  an  Haversian  system  more  highly  magnified,  h,  Haversian  canal;  I,  lacuna — 
the  lacunae  connected  by  canaliculi;  a,  artery;  v,  vein;  la,  bony  lamella.  (From 
Galloway.) 

at  a  distant  point  and  only  the  tendon  crosses  the  joint.  Note, 
for  example,  that  the  muscles  of  the  fingers  are  located  in  the  fore- 
arm, and  the  tendons  can  be  traced  across  the  wrist  and  knuckle 
joints.  The  proximal  point  of  attachment  of  a  muscle  is  its 


FIG.  96. — Connective  tissue,  showing  fibrous  structure  and  a  few  scattering  cells. 


"origin," 


the  distal  point  of  attachment  the  " insertion." 
The  middle,  thicker  portion  of  a  typical  muscle  (like  the 
biceps)  is  the  "  belly."  The  muscle  is  composed  of  muscle 
fibres,  arranged  in  bundles.  The  fibers  of  each  bundle  are 


1 82  ANIMALS 

bound  together  by  connective  tissue,  and  the  bundles  are  bound 
in  the  same  way  to  form  the  muscle  as  a  whole. 

430.  When  a  muscle  contracts  it  becomes  shorter  but  pro- 
portionally thicker,  so  that  its  volume  is  not  changed.     At 
the  time  of  contraction  it  also  undergoes  electrical  and  chemical 
changes,  and  heat  is  evolved.     These  subjects  are  discussed 
elsewhere. 

431.  The  cause  of  a  muscular  contraction  is  in  every  case  a 
stimulus.     Chemical  stimuli,  like  salts  and  acids,  applied  to  the 
muscle,  will  cause  a  contraction.     The  electric  current  will  do 
the  same.     But  the  normal  stimulus  for  the  body  musculature 
of  the  higher  animals  is  a  nerve  impulse  originating  in  some 
other  part  of  the  body.     The  origin  of  this  impulse  is  in  every 
case  to  be  traced  to  some  peripheral  sense  organs.     In  some 
cases  the  impulse  may  seem  to  arise  in  the  central  nervous 
system,  but  a  careful  analysis  will  show  that  even  in  these 
cases  the  impulse  can  be  traced  backward  to  some  sense  organ. 

432.  The  glands,  luminescent  organs  and  electrical  organs 
are  discussed  elsewhere. 

433.  We  will  next  consider  the  means  by  which  connection 
is  made  between  the  sense  organs  and  the  organs  of  response. 

THE  NERVOUS  SYSTEM 

.434.  In  no  respect  do  the  highest  animals  diverge  so.  greatly 
from  the  lowest  as  in  the  way  they  respond  to  stimuli.  This 
difference  is  due,  not  so  much  to  the  differences  between  the 
sense  organs  or  the  organs  of  response,  as  to  the  way  in  which 
the  two  sets  of  organs  are  related.  In  amoeba  the  organs  of 
sensation  and  response  are  identical,  and  no  system  of  communi- 
cation is  required,  but  in  mammals  the  organs  of  communication, 
the  brain  and  spinal  cord,  exceed  in  complexity  all  other 
organs  of  the  body  combined. 

435.  We  have  seen  that  the  muscle  fibres  of  hydra  are  parts 


NERVE   CELLS  183 

of  cells  which  are  otherwise  undifferentiated  and  exposed  at  the 
surface.  Here,  then,  the  stimulus  received  by  one  part  of  a  cell 
is  transmitted  to  another  part  of  the  same  cell.  In  other 
Ccelenterates,  some  of  these  epithelial 
cells  are  prolonged  at  the  deeper  ex- 
tremity into  slender  fibres,  which  are 
regarded  as  nerve  fibres  instead  of 
muscle  fibres.  In  this  case  the  en- 
tire cell  is  nervous  in  function,  it 
receives  and  transmits  stimuli. 
Again,  cells  are  found  which  do  not 
reach  to  the  surface  of  the  epithelial  FIG-  ^--Diagram  of  nerve 

cells   found   beneath  the  ecto- 

layer  to  which  they  belong,  and  are,    derm  of  a  jelly-fish, 
therefore,     probably     not     sensory. 

These  cells,  however,  are  multipolar  nerve  cells.  They  serve 
only  to  transmit  stimuli.  We  have,  therefore,  three  types  of 
nerve  cells: 

1.  Sensory — Transmitting — Motor. 

2.  Sensory — Transmitting. 

3.  Transmitting. 
While  in  the  taste  buds  are  found: 

4.  Sensory. 

436.  The  first  class  is  an  extremely  low  type  of  differentiation, 
and  is  not  found  in  the  sensory-motor  mechanism  of  the  higher 
animals. 

437.  In  the  jelly-fish  many  cells  of  type  three  are  found, 
and  they  are  largely  grouped  in  a  circle  of  ganglia  around  the 
edge  of  the  umbrella.     This  circle  of  ganglia  constitutes  a  simple 
central  nervous  system,  arranged  with  reference  to  the  radial 
organization  of  the  animal. 

438.  In  Annelids,  the  central  nervous  system  is  bilaterally 
arranged.     There  is  a  double  ganglion  in  each  segment,  and  all 
the  ganglia  are  connected  longitudinally  by  a  double  nerve. 


1 84 


ANIMALS 


This  chain  of  ganglia  lies  on  the  ventral  side,  just  on  the  inside 
of  the  epidermis.  The  fibres  of  the  sensory  cells  enter  the 
ganglia  through  the  paired  lateral  nerves,  which  spring  from 
each  ganglion  and  pass  out  to  all  parts  of  the  corresponding 


FIG.  98. 


FIG.  99. 


FIG.  98. — Diagram  of  the  nervous  system  of  nereis.  A,  The  brain,  ventral 
nerve  cord,  and  the  five  nerves  of  a  metamere;  B,  diagram  of  a  parapodium  to 
show  the  chief  branches  of  nerve  II.  Br,  Brain;  C,  circum-cesophageal  con- 
nectives; E,  eye;  N,  nerves;  Oe,  oesophagus;  Par,  parapodium;  P.gn,  parapodial 
ganglion;  S.g.,  segmental  ganglion;  V.N.,  ventral  nerve  cord. 

FIG.  99. — Diagram  to  show  the  relation  of  sensory  fibres  (S.F.),  motor  fibres 
(M.F.)  and  association  fibres  (A.F.)  in  a  simple  type  of  central  nervous  system. 
Ep,  epidermis;  Gl,  ganglion;  M ,  muscle;  V.N.C.,  ventral  nerve  cord. 

segment.  The  ganglia  contain  two  classes  of  cells:  i.  Those 
which  send  fibres  up  and  down  the  chain  to  other  ganglia,  but 
do  not  pass  out  of  the  central  system,  and  2,  those  which  send 
fibres  out  to  the  other  organs  through  the  lateral  nerves.  In 


NERVOUS    SYSTEM  185 

addition  to  the  ventral  nerve  chain,  the  worm  has  a  ganglion 
which  is  not  duplicated  and  which  serves  as  a  centre  for  the 
entire  system.  This  ganglion  is  located  in  the  cephalic  lobe, 
on  the  dorsal  side  of  the  oesophagus.  It  is  called  the  brain  or 
supra-cesophageal  ganglion.  It  is  connected  with  the  ventral 
chain  by  a  pair  of  connecting  nerves,  which  pass  around  the 
oesophagus  and  unite  with  the  first  ventral,  or  sub-cesophageal 
ganglion.  The  nerves  from  the  eyes,  tentacles,  cirri  and  other 
special  sense  organs  of  the  head  region,  connect  with  the  brain. 
The  central  nervous  system  of  Worms  is  usually  sharply  marked 
off  from  surrounding  tissues,  and  is  enveloped  in  a  special 
protective  connective  tissue. 

439.  The  nerve   elements   are  well  differentiated  and  fall 
naturally    into    three    distinct  classes:     i.  The  sensory  cells 
(see  page  360),  which  are  called  receptors,  always  lie  outside  the 
central  nervous  system,  and  send  a  fibre  into  the  ganglion  of  the 
corresponding  segment.     2.  The  connecting  fibres,  called  also 
association  fibres,  lie  wholly  within  the  central  nervous  system 
and  serve  as  a  connection  between  the  cells  of  the  first  and  third 
classes.     The  cells  of  these  fibres  lie  in  the  ganglia.     3.  Cells  of 
the  third  class  lie  in  the  ganglia,  but  send  fibres  out  to  the  mus- 
cles, glands  and  other  organs  of  response.     These  cells  are 
called  effectors. 

440.  Where   the   receptor  fibres   end  in   the  ganglia,  they 
divide  into  a  tuft  of  small  branches,  which  end  in  contact  with 
similar  branches  of  other  elements  or  with  the  body  of  another 
cell.     The  fibres  of  related  cells  are  also  often  intimately  con- 
nected where  no  branches  occur.     In  this  way,  physiological 
connection   is   made   between   nerve   cells   and   the   stimulus 
transmitted  from  one  to  another.     The  effectors,  where  they 
end  on  muscles  or  glands,  also  break  up  into  numerous  small 
branches,  which  terminate  in  small  disc-like  enlargements  in 
contact  with  the  response  organs. 

441.  The  mechanism  of  response  is  then  as  follows:  When  a 


i86 


ANIMALS 


sense  organ  is  sufficiently  stimulated  a  process  is  set  up  which 
travels  along  the  receptor  element  to  the  central  nervous 
system.  Here  connection  may  be  made  directly  with  an  effec- 
tor element,  and  the  stimulus  pass  out  on  another  fibre  to  a  mus- 
cle or  other  response  organ  and  bring  the  latter  into  action. 
Or,  the  stimulus  may  be  taken  up  by  an  association  element, 


FIG.  ioo. — A  photomicrograph  of  a  section  through  the  ventral  edge  of  the 
brain  of  the  crayfish.  The  black  line  represents  the  plane  of  symmetry.  On 
either  side  of  it  are  groups  of  ganglionic  cells  and,  farther  out,  part  of  the  circum- 
cesophageal  connectives  in  cross  section.  Among  the  larger  cells  may  be  dis- 
tinguished at  least  six  pairs  of  cells.  The  two  cells  of  each  pair  are  alike  in 
size  and  exactly  symmetrical  in  position  and  presumably  their  functions  are 
identical  with  regard  to  the  corresponding  side  of  the  body.  This  example 
seems  to  indicate  that  within  the  nervous  system  differentiation  may  extend 
to  individual  cells. 

and  through  it  transferred  to  an  effector  of  the  same  or  a  distant 
ganglion.  Thus  a  response  may  occur  in  a  distant  portion  of 
the  body.  A  stimulus  originating  in  the  periphery  may  first 
pass  to  the  brain,  and  from  there  return  to  the  organs  of  response. 
442 .  The  superiority  of  the  nervous  system  of  the  worm  over 
that  of  the  Ccelenterate  is  evident  in  the  more  complete  dif- 


NERVOUS    SYSTEM 


i87 


FIG.  101. — Nerve  cell  and  striate  muscle  fibre.  A,  Nerve  cell;  g,  the  body  of 
the  cell — a  ganglionic  cell;  d,  dendron  or  dendritic  process;  a.x.,  axis  cylinder 
process  or  nerve  fibre.  At  n.f.  the  fibre  is  covered  by  a  medullary  sheath; 
n.m.,  the  ending  of  the  nerve  fibre  on  a  muscle  fibre.  D,  Part  of  nerve  fibre 
highly  magnified;  a,  axis  cylinder;  m,  medullary  sheath;  s,  sheath  of  Schwann; 
n,  node;  m.f  muscle  fibre;  /,  muscle  fibril.  B,  Muscle  fibril  highly  magnified; 
C,  the  same  in  contracted  condition.  (From  Galloway.) 


1 88  ANIMALS 

f erentiation  of  the  elements,  but  the  more  significant  difference 
is  the  centralization.  The  brain  of  the  worm  has  no  counter- 
part in  the  medusae  where  there  are  a  number  of  centres  of  co- 
ordinate rank. 

443.  The  central  nervous  system  of  Arthropods  is,  in  most 
respects,  much  like  that   of   Worms.     The   most   significant 
difference  is  the  better  development  of  the  brain,  which  results 
largely  from  the  better  development  of  the  special  sense  organs. 
The  differentiation  of  body  segments  and  of  the  appendages 
also  makes  possible  a  much  greater  number  of  movements,  and, 
therefore,  demands  greater  complexity  in  association  elements 
of  the  nervous  system.     Besides,  the  nervous  system  is  itself 
subject    to   independent    differentiation,    and    this    manifests 
itself  in  the  increased  complexity  of  the  association  elements. 
This  takes  place  especially  in  the  brain  or  highest  centre,  and 
it  is  in  this  respect  especially  that  the  brain  of  the  crayfish 
and  insect  is  in  advance  of  that  of  the  worm. 

444.  The  central  nervous  system  of  the  Vertebrates  is  wholly 
dorsal  and  consists  of  the  brain  and  spinal  cord.     It  originates 
from  the  ectoderm  as  a  tube,  but  the  cavity  of  the  tube  remains 
extremely  small,  except  in  the  region  of  the  brain,  where  a  series 
of  chambers  of  considerable  size  are  developed.     The  nerve 
elements  develop  in  the  walls  of  this  tube  and  form  a  continu- 
ous ganglionic  mass.     The  nerves  are  arranged  segmentally, 
but  there  is  little  evidence  of  segmentation  in  the  brain  or  spinal 
cord.     In  a  cross  section  of  the  spinal  cord  the  nerve  cells  are 

•seen  to  be  massed  in  the  central  portion  in  an  area  resembling 
a  letter  H.  The  space  around  this  is  made  up  of  longitudinal 
nerve  fibres,  which  are  each  one  encased  in  a  thick  sheath  of  a 
fatty  substance,  which  gives  them  an  opaque  white  appearance 
when  seen  in  mass.  The  central  gray  area  contains  naked 
fibres  as  well  as  cells.  In  the  central  parts  of  the  brain  there  are 
also  large  masses  of  the  cellular  gray  matter,  but  a  still  larger 
quantity  of  the  gray  is  distributed  over  the  surfaces  of  the  folded 


THE  BRAIN 


FIG.  102. — Brain  of  a  shark,  Notidanus.     A,  Dorsal  view;  B.  lateral  view.     The 
roots  of  the  nerves  are  represented  in  black.     (From  Johnston.) 


I QO  ANIMALS 

cerebral  and  cerebellar  regions.  Hence,  the  surface  of  the  brain 
is  largely  .composed  of  the  gray  matter,  while  the  fibre  tracts 
are  wholly  beneath  the  surface.  In  the  human  body  there  are 
forty-three  pairs  of  nerves,  twelve  connecting  with  the  brain 
and  thirty-one  with  the  spinal  cord.  A  typical  spinal  nerve 
is  connected  with  the  spinal  cord  by  two  "roots,"  one  dorsal 
and  one  ventral.  On  the  dorsal  root,  not  far  from  the  spinal 
cord,  there  is  a  ganglion,  and  immediately  beyond  the  two  roots 
unite  to  form  the  spinal  nerve.  The  latter  then  divides  into 


FIG.  103. — Diagram  of  a  cross  section  of  the  spinal  cord  and  the  roots  of  the 
spinal  nerves.  C,  Central  canal;  df,  dorsal  fissure;  dr,  dorsal  root  of  spinal  nerve, 
arising  from  the  dorsal  horn  of  the  gray  matter  (g) ;  gn,  ganglion  on  the  dorsal 
root;  n,  spinal  nerve;  a/,  ventral  fissure;  vr,  ventral  root  of  the  spinal  nerve, 
arising  from  the  ventral  horn  of  the  gray  maater;  w,  white  matter.  (From 
Galloway,  by  Folsom.) 

three  main  trunks;  one  passes  into  the  body  cavity  to  connect 
with  the  sympathetic  system  which  supplies  the  viscera,  one 
passes  up  to  the  muscles  and  skin  of  the  dorsal  portion  of  the 
body,  and  the  last  turns  downward  to  the  muscles  and  skin 
of  the  ventral  portion  of  the  body.  Both  dorsal  and  ventral 
branches  contain  both  receptor  and  effector  fibres,  but  at  the 
juncture  of  dorsal  and  ventral  roots  the  two  classes  of  fibres 
separate,  the  effectors  pass  over  the  ventral  root  and  the  recep- 
tors pass  over  the  dorsal  root.  The  cells  of  the  receptors  lie 
in  the  spinal  ganglion  of  the  dorsal  root,  while  the  cells  of  the 
effectors  lie  in  the  ventral  horn  of  the  gray  matter  in  the  cord. 


CRANIAL   NERVES  IQI 

445.  The  spinal  nerves  are  quite  uniform  in  their  relations 
so  far  as  described  above,  but  the  cranial  nerves  are  consider- 
ably modified.     The  olfactory  (I)  and  optic  (II)  nerves  are  not 
comparable  with  spinal  nerves  at  all.     The  III,  IV,  and  VI  go 
to  the  muscles  of  the  eye  ball  and  contain  no  receptor  elements. 
The  VIII  nerve  is  the  auditory  and  is  purely  receptor.     The 
V  and  VII  nerves  supply  the  skin  and  muscles  of  the  face  and 
lower  jaw,  and  are  mixed  in  function.     The  IX  nerve  supplies 
the  muscles  and  sense  organs  of  the  tongue  (taste)  and  pharynx, 
and  is  also  mixed.     The  X  nerve  supplies  the  viscera  from  the 
pharynx  to  the  liver,  including  larynx,  lungs,  oesophagus  and 
stomach  and  heart.     It  is  also  a  mixed  nerve.     The  XI  and 
XII  are  chiefly  effector  in  function;  they  supply  chiefly  muscles 
of  the  neck  region. 

ENERGY  RELATIONS  OF  THE  ANIMAL 

446.  When  an  animal  puts  itself  in  motion,  work  is  being 
done,  as  is  the  case  when  any  other  body  is  being  moved,  and 
when  work  is  being  done  there  is  an  expenditure  of  energy. 
However,  throughout  its  life  the  animal  is  continually  moving 
itself  as  well  as  other  bodies,  and  hence,  as  constantly  expending 
energy.     And  for  a  considerable  portion  of  its  life  its  capability 
of  expending  energy  increases,  even  though  energy  is  constantly 
being  spent.     Now,  a  fundamental  postulate  of  physics  says 
that  energy  is  never  created,  but  that  wherever  it  appears  it 
has  merely  been  transformed  or  transferred  from  some  other 
source.     The  animal  may  be  exhausted  temporarily  and  yet 
after  a  while  its  power  of  expending  energy  is  renewed.     And 
we  know  that  the  condition  upon  which  this  renewal  of  energy 
depends  is  the  supply  of  proper  food  to  the  organism.     Thus 
the  food  is  apparently  the  energy  source  for  the  animal. 

447.  If  we  analyze  the  foods  of  animals  we  find  that  the  most 
important  by  far,  for  their  energy-yielding  value,  as  food,  are 


1 92  ANIMALS 

TABLE  OF  FOOD  VALUES  (FOR  THE  HUMAN  BODY). 

Available  constituents. 


Food 
substances. 


%  Proteid. 


Fats  %. 


Beef  

19-4 

i  —                 ~ 

Veal  
Mutton        .... 

16.5 

Pork    

16.8 

,  

Chicken 

2O   A. 

Fish  

20.5 

-            0 

Ham  

Coif  rinrk 

23-0  . 
8  i 

T  .,  f. 

Sausage  

15.0 

Effgs 

12.  2 

«_  _~~                                                                    __~— 

Milk 

3  •  2 

/"*r 

Cheese  

27.4 

T  arrl 

-3 

Beans  

16.8 

_  

Rice 

6    A 

u-4 

Flour  (wheat)  .  . 

8.7 

— 

Oatmeal  

IO.O 

—                                                                             — 

Wheat  bread  .  .  . 

5-7 

— 

Potatoes  (Irish). 

1.5 

. 

Cabbage  

1.3 

Turnips  

•  7 

Asparagus  

i-5 

- 

Peas  (green)  .... 

4-5 

- 

Lettuce  

I  .0 

i 

Apples.  .  . 

t 

Sugar  

.0 

Olive  oil  

.0 

FOODS 


193 


Available  constituents. 


Food 
substances. 


Carbohydrates. 


Water,  %. 


Ttaaf 

71    "% 

rseei  

7  C    O 

Veal  

.... 

76  o 

Mutton  

T>/M-tr 

60  o 

rOrK  

Fish 

76  •  b 
64  o 

Ham 

Salt  pork  
Sausage        .... 



25.5 
IO.O 

Eggs  

4x-5 

Milk 

4Q 

72.2 

Q7     A 

2.  I 

°/  -4 

Butter  
Lard  

•  5 
44.0 



14.0 

•  7 

Rice 

77.0 

L£-  0 

Flour  (wheat)..  , 

73.6 

"•  5 

64.0 

Wheat  bread  .  . 

56.0 

Potatoes  (Irish) 

20.  o 

3o-  7 

Cabbage  

4.2 

75-° 
90  o 

Turnips 

5-5 

OO    t» 

Asparagus  

2.  2 

Q2      C 

IO.O 

y.5  •  o 

Lettuce        .    . 

i-5 

1  1  •  j 

Apples  

n-3 

y4-  o 

Q  {•       Q 

Sugar.  .  . 

99.0 

Olive  oil  

1.0 

194  ANIMALS 

the  carbohydrates  and  fats.     We  also  know  that  these  sul 
stances  may  be  readily  made  to  yield  energy  by  heating  thei 
in  the  presence  of  oxygen.     The  carbohydrates  are  then  decom- 
posed and  their  constituents  unite  with  oxygen  to  form  carbon- 
dioxide  and  water.     This  union  produces  heat,  which  is  a  form 
of  energy. 

448.  The  wood  in  the  firebox  of  an  engine  is  a  case  where 
C6Hio05+602  =  6C02+5H2O.     The    heat    liberated    in    this 
process  is  taken  up  by  the  water,  which,  -because  of  the  added 
energy,  expands  into  stem.     The  energy  of  the  enclosed  steam 
is  evident  in  the  pressure  which  it  exerts,  and  by  means  of  the 
mechanism  of  the  engine,  the  pressure  energy  is  transformed 
into  the  motion  of  piston,  crankshaft  and  wheels.     Thus  the 
oxidation  of  the  fuel  releases  the  energy  which  causes  the 
engine  to  move  and  do  work.     Fats  and  oils,  CmHn(O)  when 
burned  yield  C02  and  H20  and  energy  is  also  set  free  in  the 
same  way. 

449.  That  in  these  processes  it  is  a  question  of  liberation 
rather  than  the  creation  of  energy  becomes  clear  if  we  consider 
the  origin  of  the  substances.     The  carbohydrates,  we  will  recall, 
were  formed  in  the  leaf  from  CO2  and  H2O,  through  the  agency 
of  chlorophyll  and  light.     But  light  is  a  form  of  energy  whose 
source  is  the  sun.     In  some  way  the  energy  of  the  ether  vibra- 
tions breaks  up  the  C02  molecule  and  the  carbon  atom  unites 
with  the  H20  molecule,  producing,  as  a  final  result,  starch 
(Ce  HIQ  65),  sugar,  cellulose,  oil,  proteid  or  other  carbon  com- 
pound.    These  substances  remain  stable  at  ordinary  tempera- 
ture, but  when  slightly  heated  they  break  down,  and  the  C  ther 
unites  with  the  0  and  the  energy  which  was  stored  up  in  photo- 
synthesis is  again  set  free.     In  physical  terms,  the  energy  oi 
the  light  becomes  potential  energy  in  the  chemical  compound 
and  again  kinetic  energy  of  heat  in  combustion. 

450.  The  processes  by  which  the  food  yields  energy  to  the 
animal  are  closely  analogous  to  the  case  of  the  fuel  in  the 


SOURCE    OF   ENERGY  1 95 

steam  engine,   but  it  will  be  necessary   to   point  out  some 
differences  and  show  how  the  animal  engine  works. 

451.  In  the  first  place,  the  animal  "fire  box"  is  not  the 
stomach  or  lungs  or  any  similar  organ.     The  combustion  takes 
place  in  the  cells  and  each  individual  cell  of  the  animal  body  is 
a  unit  so  far  as  this  process  is  concerned.     Some  cells  require 
more  fuel  than  others  in  proportion  as  some  are  greater  workers 
than  others.     Consequently  the  muscle  cells  require  much  fuel. 
But  how  does  the  food  get  to  the  muscle  cells?     That  is  another 
question,  and  to  make  it  clear  we  will  return  to  the  case  of  the 
amceba. 

452.  Digestion. — When  the  amceba  comes  in  contact  with  a 
particle  of  food  its  protoplasm  flows  around  the  particle  until 
it  is  entirely  enclosed  and  lies  embedded  in  the  protoplasm, 
but  with  the  particle  there  is  also  engulfed  a  droplet  of  water. 
This  is  called  a  food  vacuole.     The  water  in  which  the  amceba 
lives  is  always  slightly  alkaline  and  the  protoplasm  of  the  amceba 
is  also  alkaline,  but  if  delicate  test  is  made  it  is  found  that  the 
water  or  fluid  of  the  food-vacuole  becomes  slightly  acid  and 
soon  changes  may  be  observed  in   the  food  particle.     If  it 
was  a  living  object  it   soon   dies;   if   it   was   a   blue   green 
alga,   the  blue  color  is  rapidly  diffused  into  the    surround- 
ing medium.     The  vacuole  becomes   alkaline,   and   the   food 
substance    becomes    translucent    as    though    it    were    being 
dissolved,  and  gradually  it  disappears.     Probably  some  por- 
tions remain  unchanged.     The  vacuole  grows  smaller  until  the 
unchanged  portions  of  the  ingested  object  are  closely  surrounded 
by  protoplasm.     Finally,  what  remains  in  the  food  vacuole  is 
ejected.     While  this  has  been  going  on,  other  food  vacuoles 
have  been  formed  and  the  same  series  of  phenomena  take  place 
in  each.     All  the  while  the  animal  is  growing  larger  at  the  ex- 
pense, evidently,  of  the  substance  of  the  food- vacuoles.     It  is 
important  to  note  that  the  food  substance  of  the  vacuoles 
disappears  and  later  reappears  as  protoplasm.     Between  these 


1 96  ANIMALS 

two  stages  there  is  a  stage  when  this  substance  is  invisible  in 
solution,  and  it  is  during  this  stage  that  it  passes  from  the 
vacuole  into  the  protoplasm. 

453.  Let  us  recall  the  phenomena  of  fermentation  as  exhib- 
ited by  the  yeast  plant  or  bacteria.     Here  we  have  living  cells  en- 
closed in  a  membrane,  through  which  no  visible  particle  is  known 
to  pass.     The  bacteria  live  in  a  watery  medium  surrounded 
by  solid  substances,  upon  which  they  are  nourished.     In  the 
medium  there  appear  substances  which  are  secreted  by  the  liv- 
ing cells  and  which  act  upon  the  food  substances  in  such  a  way 
as  to  cause  them  to  go  into  solution.     This  process  may  come 
about  in  many  different  ways,  but  the  result  is  always  a  solution 
which  may  be  absorbed  by  the  cell  through  the  membrane. 
The  chief  difference  between  the  bacteria  and  the  amoeba,  with 
respect  to  the  way  in  which  the  food  is  prepared,  so  that  it 
may  be  absorbed,  is  this:  The  bacteria  fill  the  surrounding 
medium  with  an  enzyme  which  dissolves  the  food  substances 
there.     The  amoeba  takes  into  its  body  a  droplet  of  the  medium 
containing  a  particle  of  food,  and  into  this  droplet  of  the  medium 
it  secretes  a  digestive  fluid.     We  may  transpose  terms  and  say 
that  the  bacteria  digest  the  food  before  it  is  taken  into  the 
body  and  the  amoeba  carries  on  a  process  of  fermentation 
within  its  food-vacuoles.     That  is  to  say,  digestion  is  a  matter 
of  fermentation. 

454.  The  term  gastro-vascular  cavity  applied  to  the  central 
cavity  of  hydra  indicates  that  it  is  analogous  to  the  stomach, 
and  hence,  concerned  in  digestion.     The  food  unquestionably 
passes  into  this  cavity,  but  to  what  extent  it  is  there  digested  is 
uncertain.     Small  particles  are  known  to  be  captured  by  the 
flagellate  cells  of  the  entoderm  and  engulfed  by  the  protoplasm. 
So  that  in  this  case  as  well  as  in  Sponges  and  some  Flat-worms 
the  digestion  resembles  that  of  amoeba.     In  this  case  the  func- 
tion of  the  gastro-vascular  cavity  would  be  to  serve  as  a  sort  of 
trap  for  the  food  particles.     But  frequently  objects  are  captured 


DIGESTION 


197 


and  swallowed  which  are  much  too  large  to  be  taken  up  by  a 

cell.     In  the  sea  anemone  such  objects  undergo  partial  dis- 

integration in  the  gastro-vascular  cavity,  and  the  fragments 

are  taken  up  by  the  cells.     The  hydra  lacks 

the  organs  by  which  this  is  accomplished  by 

the  anemone,  but  it  is  still  probable  that  the 

close  application  of  the  walls  of  hydra  to  the 

prey  may  accomplish  the  same  end.     The 

undigested  portions  of  the  food  are  cast  out 

at  the  mouth. 

455.  The  elongated  form  of  the  body  of 
the  worm  makes  possible  a  considerable  ad- 
vance in  the  digestive  system.  The  digestive 
cavity  is  a  slender  tube  opening  to  the  ex- 
terior at  each  end.  The  food  is  taken  in  at 
the  mouth,  and  as  it  passes  slowly  along  the 
narrow  channel  it  is  gradually  digested  and 
absorbed.  The  parts  that  remain  undigested 
are  cast  out  at  the  vent.  The  elongated 
form  makes  possible  the  successive  applica- 
tion of  different  agencies  of  digestion  to  a 
given  particle  and  the  simultaneous  opera- 

FIG.  104.—  The  in- 

tion  of  these  agencies  in  different  parts  of    testine  of  a   worm 


the  canal.      In  nereis   there  are  jaws  and 

denticles  by  which  the  food  is  captured  and  ing  of  the  glandular 

forced  into  the  mouth,  and  perhaps,  to  some  body-wall'  "is*'  repre- 

extent,  lacerated.     There  is  a  pair  of  "sail-  ^ed«i5    outlife' 

B.C.,    Body    cavity; 

vary"  glands  which  open  into  the  anterior    C.E.,  glandular  epi- 
end  of  the  digestive  tract  and  throughout    ^um'}  Int}  l 
the  remainder  of  its  length  the  intestinal  epi- 
thelium is  thickly  studded  with  unicellular  glands,  which  also 
pour  a  secretion  into  the  digestive  cavity.     In  the  earthworm 
the  digestive  canal  is  more  differentiated. 
456.  In  Worms  we  have  unquestionably  a  case  of  a  true 


198  ANIMALS 

digestive  cavity,  into  which  the  digestive  enzymes  are  se- 
creted, in  which  digestion  takes  place  and  from  which  the 
soluble  products  are  then  absorbed.  The  long  digestive  tube 
gives  a  large  absorbing  surface,  but  in  some  cases,  as  in  the 
earthworm,  the  surface  is  further  increased  by  a  longitudinal 
fold  which  hangs  from  the  dorsal  side  of  the  canal  and  gives  the 
lumen  of  the  canal  a  crescentic  form  in  cross  section.  The 


FIG.  106. 


FIG.  105.  FIG.  107. 

FIG.  105. — Cross  section  of  the  intestine  of  nereis  showing  the  glandular 
epithelium  and  blood  capillaries  (black). 

FIG.  106. — A  part  of  the  preceding  figure  enlarged.  The  upper  two-thirds 
of  the  figure  is  the  epithelium.  Below  that  is  a  blood  capillary.  Then  follows 
a  layer  of  longitudinal  muscle  fibres  cut  across  and  a  layer  of  circular  muscle 
fibres  lying  in  the  plane  of  the  section.  The  lower  layer  is  an  extremely  thin 
epithelium  lining  the  outer  surface  of  the  intestine. 

FIG.  107. — A  surface  view  of  the  inner  surface  of  the  intestinal  epithelium. 
The  cells  are  outlined  by  a  network  of  supporting  fibres. 

salivary  glands  are  a  simple  type  of  a  compound  gland.  The 
glandular  epithelium  is  pushed  outward  into  the  body  cavity 
and  is  greatly  folded  so  that  a  large  glandular  surface  occupies 
a  small  space.  The  part  by  which  the  gland  is  connected  with 
the  intestine  forms  a  duct  through  which  the  secretion  is  poured 
into  the  digestive  cavity.  In  nereis  the  greater  part  of  digestion 
is  doubtless  due  to  the  activity  of  the  unicellular  glands. 
457.  The  glandular  intestinal  epithelum  is  only  a  lining  of  a 


DIGESTION 


199 


tube  which  is  composed  largely  of  muscle  fibres.  The  muscles 
by  peristaltic  contraction  force  the  contents  of  the  canal  slowly 
backward.  They  also  regulate  the  size  of  the  canal  as  the 
volume  of  the  contents  may  demand. 


FIG.  1 08. — Wall  of  the  intestine  in  a  small  aquatic  Annelid,  Chaetogaster- 
There  are  only  two  thin  layers  of  cells,  one  (a)  which  forms  the  lining  of  the 
body  cavity  and  (b)  the  intestinal  epithelium. 

458.  The  separation  of  the  digestive  processes  advances  a 
step  farther  in  Arthropods  and  the  digestive  tract  is  divided 
into  well-defined  regions.  In  the  crayfish  there  are,  in  the 
immediate  region  of  the  mouth,  six  pairs  of  segmental  appen- 
dages which  are  modified  for  grasping  and  tearing  up  the  food. 


FIG.  109. — A  section  through  one  of  the  folds  of  the  intestinal  epithelium  of 
nereis,  showing  a  few  of  the  glandular  cells.  The  inner  ends  of  the  greatly 
elongated  cells  are  filled  with  a  granular  secretion.  The  accumulation  of  the 
secreted  substance  in  the  ends  of  the  cells  causes  them  to  swell  and  hence  throws 
the  surface  into  folds. 

From  the  mouth  a  short  cesophagus  leads  into  a  large  muscular 
stomach,  which  consists  of  two  divisions.  The  first  is  lined 
with  chitin  and  is  provided  with  a  mechanism  consisting  of 
several  chitinous  hooks  or  teeth  and  a  set  of  muscles  for  oper- 
ating them.  By  means  of  this  the  food  is  still  further  broken 
up.  A  pair  of  large  digestive  glands  lying  in  the  body  cavity 


20O 


ANIMALS 


communicate  with  the  second  smaller  portion  of  the  stomach 
by  means  of  short  ducts.  Digestion  proper  takes  place  in  this 
portion  of  the  digestive  tract.  A  narrow  intestine  of  simple 
structure  leads  to  the  vent  at  the  posterior  end  of  the  abdomen. 
459.  The  function  of  salivary  glands  is  primarily  to 
moisten  the  food  preparatory  to  swallowing.  Consequently 
they  are  only  necessary  in  terrestrial  animals.  The  so-called 
salivary  glands  of  nereis  owe  their  name  to  their  position  and 


FIG.  no. — Digestion  and  circulatory  systems  of  the  crayfish.  Upper  figure: 
a,  Mouth;  ,b  oesophagus;  c,  cardiac  portion  of  stomach;  d-e,  pyloric  portion  of 
stomach;  e,  opening  of  digestive  gland;  /,  intestine;  g,  vent;  h,  digestive  gland 
("liver");  i,  heart;  j,  gonad;  k,  brain;  /,  /,  ventral  nerve  cord.  Lower  figure: 
a,  Heart;  b,  dorsal  abdominal  artery;  c,  sternal  artery  which  branches  into  the 
ventral  abdominal  and  the  ventral  thoracic  arteries;  d,  ophthalmic  artery; 
e,  antennary  artery;  /,  hepatic  artery;  g,  blood  sinuses;  h,  afferent  branchial 
vessels;  i,  efferent  branchial  vessels. 

must  not  be  supposed  to  be  in  any  sense  true  salivary  glands. 
No  such  glands  occur  in  the  crayfish,  but  in  the  terrestrial 
Arthropods,  the  Insects,  they  are  generally  found.  The  mouth 
parts  in  Insects  consist  of  three  pairs  of  appendages.  There 
is  an  oesophagus,  into  which  the  salivary  glands  open.  Some- 
times the  oesophagus  is  enlarged  to  form  a  crop.  Sometimes 
there  is  also  a  gastric  mill  analogous  to  that  of  the  crayfish. 
Then  follows  the  true  stomach  which  has  numerous  small 


DIGESTION  201 

glands  embedded  in  its  thick  walls  or  else  there  are  larger 
glands  lying  outside  the  stomach  wall,  but  connected  with  it  by 
ducts.  From  the  stomach  the  digestive  tract  continues,  first 
as  a  slender  "small  intestine"  which  farther  on  expands  into  a 
wider  " large  intestine."  The  latter  ends  at  the  vent. 

460.  Quite  generally  the  digestive  tract  of  Vertebrates  is 
differentiated  into  the  following  series  of  parts;  buccal  cavity, 
oesophagus,  stomach,  small  intestine,  and  large  intestine.     Its 
walls  are  very  muscular,  especially  those  of  the  stomach  and 
small  intestine.     The  internal  surface  area  is  greatly  increased 
by  folds  and  countless  minute  thread-like  elevations,  the  villi. 
The  teeth  with  which  the  mouth  is  usually  armed  serve  either 
for  seizing  and  swallowing  the  prey  or  for  mastication. 

461.  The  digestive  glands  are  numerous  and  large.     In  man 
there  are  three  pairs  of  large  salivary  glands,  besides  a  number 
of  smaller  ones,  which  open  into  the  buccal  cavity.     But  for 
Vertebrates  the  general  statement  also  applies,  that  salivary 
glands  are  characteristic  only  of  terrestrial  forms.     Besides 
moistening  the  food  for  swallowing  it  the  saliva  also  sometimes 
seems  to  soften  it  preparatory  to  digestion  (birds).     In  herbiv- 
orous animals  the  saliva  often  contains  an  amylolytic  ferment, 
ptyalin.     Embedded  in  the  thick  walls  of  the  stomach   are 
numerous   small  tubular  glands  which  secrete  gastric  fluid. 
This  contains  hydrochloric   acid  and  a  proteolytic  ferment, 
pepsin. 

462.  A  gland  of  considerable  size,  the  pancreas,  opens  into 
the  intestine  near  the  stomach.     Its  secretion  contains  three 
ferments,  one,  amylopsin,  is  amylolytic;  another,  trypsin,  is 
proteolytic,  and  a  third,  steapsin,  decomposes  fats  into  glycerine 
and  fatty  acids.     There  are  also  numerous  small  glands  em- 
bedded in  the  wall  of  the  intestine,  which  are  said  to  secrete  the 
ferment,  invertin,  which  is  found  in  the  intestine  and  which 
inverts  maltose  into  glucose. 

463 .  Amylolytic  ferments  are  not  all  alike.     That  is,  there  are 


202 


• 

ANIMALS 


Duo 


C.B.D.- 


FIG.  in. — Diagram  of  the  digestive  tract  of  man.     A.C.,  Ascending  colon; 
C,  cardiac  portion  of  the  stomach;  C.B.D.,  common  bile  duct;  Ca,   caecum; 


DIGESTION  203 

a  number  of  substances,  derived  from  different  sources,  which 
have  the  power  of  changing  starch  into  sugar,  but  produce  this 
result  under  different  conditions.  This  indicates  a  difference 
in  constitution  of  the  ferments.  The  same  is  true  of  the  pro- 
teolytic  ferments,  for  example,  pepsin  and  trypsin  are  both 
proteolytic,  but  the  one  in  acid  media,  the  other  in  alkaline, 
and  in  Cephalopods  there  is  a  ferment  which  resembles  both  of 
these.  Generally,  in  higher  animals,  there  are  more  kinds  of 
ferments,  but  each  is  more  circumscribed  in  its  action.  Con- 
versely, in  the  lower  forms,  the  ferments  are  fewer  in  kind  but 
more  general  in  action.  So  there  is  apparently  in  the  higher 
animals  a  differentiation  of  ferments  to  correspond  with  the 
structural  differentiation  of  the  digestive  tract. 

464.  In    the   food   vacuole    of    amoeba    the    medium   first 
becomes  acid  but  later,  at  the  time  when  the  food  particles  are 
disintegrating,  the  reaction  is  alkaline.     Proteolytic  and  amy- 
lolytic  ferments  are  present,  and  these  seem  to  vary  with  differ- 
ent types  of  Protozoa  and  the  exact  nature  of  those  found  in 
amoeba  is  not  certainly  known. 

465.  The  fluid  of  the  gastro-vascular  cavity  of  Ccelenterates 
has  no  amylolytic  powers.     There  is  a  slight  proteolytic  action 
which  probably  serves  to  dissociate  large  objects  so  that  the 
particles  may  be  ingested  by  the  entodermal  cells. 

466.  The  digestive  fluids  of  the  earthworm  and  nereis  are 
both  tryptic  and  diastatic.     The  earthworm  covers  leaves  it 
means  to  swallow  with  saliva  and  allows  them  to  digest  for 
some  time  before  swallowing  them. 

467.  The  digestive  gland  of  the  crayfish  has  strong  proteo- 
lytic and  amylolytic  action  in  both  acid  and  alkaline  media. 
Hence,  it  resembles  both  gastric  and  pancreatic  digestion  of 

D.C.,  descending  colon;  Duo,  duodenum;  Ep.Gl.,  epiglottis ;G.B.,  gall  bladder; 
H.D.,  hepatic  duct;  II,  ileum;  OC,  oral  cavity;  Oes,  oesophagus;  P,  pyloric  portion 
of  the  stomach;  Pa,  pancreas;  P.  D.,  pancreatic  duct;  Ph,  pharynx;  P.G.,  parotid 
gland;  R,  rectum;  S,  stomach;  S.L.,  sublingual  gland;  S.M.,  sub  maxillary  gland; 
T,  tongue;  T.C.,  transverse  colon;  Tr,  trachea;  v.A.,  vermiform- appendix. 


204 


ANIMALS 


Vertebrates.  In  the  cockroach  the  salivary  glands  have  an 
amylolytic  action.  The  intestinal  fluids  have  amylolytic,  pro- 
teolytic  and  inverting  action,  and  the  reaction  is  neutral  and 
alkaline. 

468.  Circulation. — However  simple  or  complicated  the  diges- 
tive processes  may  be,  the  result  is  essentially  the  same.     The 
end  finally  attained  is  food  substances  prepared  for  absorption. 
This  is  a  function  entirely  distinct  from  digestion,  and  since 

each  cell  must  absorb  food  for  itself, 
little  differentiation  is  to  be  looked  for 
in  connection  with  this  function.  How- 
ever, only  those  cells  can  absorb  which 
are  in  contact  with  the  food,  i.  e.,  the 
cells  lining  the  digestive  tract.  Those 
farther  removed  must  receive  their  por- 
tion from  those  nearer  the  source.  In 
hydra  no  cell  is  more  than  one  cell  re- 
moved from  the  seat  of  digestion,  for 
the  gas tro- vascular  cavity  extends  to 
all  parts  of  the  body,  even  to  the  tips 
of  the  tentacles,  and  whether  digestion 
takes  place  in  the  gastro- vascular  cavity 

or  in  the  entodermal  cells  the  ectoderm  cells  are  only  one  cell 

layer  removed. 

469.  In  the  smallest  Annelids  the  intestinal  wall  is  very  thin. 
The  same  is  true  of  the  body  wall,  and  the  two  are  separated  by 
a  space,  the  body  cavity,  which  is  filled  with  a  fluid  ("body 
fluid").     This  body  fluid  nourishes  the  tissues  bathed  by  it 
and  it  is  constantly  replenished  by  the  substances  absorbed  by 
the  intestine.     The  movements  of  the  animal  force  the  body 
fluid  about  so  that  it  becomes  throughly  mixed  and  freshly 
absorbed  matter  is  thus  directly  brought  to  the  farthest  tissues 
of  the  body. 

470.  In  the  larger  worms,  however,  the  tissues  are  often  so 


FIG.  112. — A  view  of  the 
outer  surface  of  the  intes- 
tine of  nereis,  showing  a 
network  of  blood  capillaries 
and  two  sets  of  slender  mus- 
cle fibres  crossing  each  other 
at  right  angles. 


CIRCULATION 


205 


thick  that  the  deeper  lying  cells  would  be  starved  by  such  a 
method  of  food  distribution.  Moreover,  the  wall  of  the  digestive 
tract  is  so  thick  that  it  would  greatly  impede  the  transfer  of 
absorbed  food  to  the  body  fluid.  There  is,  therefore,  necessary 
a  system  of  channels  by  which  the  food  may  more  readily  be 
transferred  from  the  seat  of  digestion  to  the  place  of  assimila- 

A 


B 


FIG.  113. — The  circulatory  system  of  annelids.  A,  A  longitudinal  section  of 
a  blood-vessel  of  a  small  fresh-water  annelid  (Chaetogaster)  showing  extremely 
thin  walls.  B,  Cross  section  diagram  of  nereis  to  show  the  arrangement  of  the 
vessels;  D.V.,  dorsal  vessel;  Int,  intestine;  N.  nephridium;  P.V.,  parapodial  ves- 
sels; V.I.,  intestinal  vessels  and  capillaries;  V.V.,  ventral  vessel.  All  vessels 
black. 

tion.  These  channels  consist  of  a  network  of  tubes  of  extremely 
small  calibre,  which  penetrate  to  every  part  of  the  intestinal 
wall,  immediately  outside  the  intestinal  epithelium.  Larger 
vessels  lead  from  this  network  of  capillaries  to  a  much  larger 
vessel  which  runs  longitudinally  along  the  mid-dorsal  line  of 
the  body.  In  each  segment  branches  of  the  dorsal  vessel  lead 
out  laterally  to  the  muscles,  epidermis  and  all  other  organs  of 


206  ANIMALS 

the  body,  where  they  divide  into  another  network  of  capillary 
vessels,  through  which  the  blood  is  distributed  to  all  the  tissues. 
From  this  second  system  of  capillaries  larger  vessels  lead  to 
another  large,  longitudinal  vessel  lying  between  the  ventral 
nerve  cord  and  the  intestine.  This  vessel  is  connected  with  the 
intestinal  system  of  capillaries,  and  thus  a  complete  circuit  is 
formed.  The  larger  vessels  are  muscular,  and  by  their  rhythmi- 
cal contraction  the  blood  is  forced  along.  This  is  especially 
true  of  the  dorsal  longitudinal  vessel,  in  which  a  continuous 
series  of  contractions  pass  forward  from  posterior  to  anterior, 
forcing  the  blood  along  in  the  same  direction.  In  the  ventral 
longitudinal  vessel  the  blood  flows  from  anterior  to  posterior. 

471.  In  the  system  just  described  the  vessels  are  "closed"; 
that  is,  they  do  not  open  into  the  body  cavity,  and  they  con- 
tain blood,  which  is  not  the  same  as  the  body  fluid.     In  many 
invertebrates,  however,  the  body  cavity  forms  a  part  of  the 
system  of  spaces  through  which  the  blood  circulates  and  in  this 
case  there  is  no  distinction  of  blood  and  body  fluid.     This  is 
the  type  of  circulatory  system  found  in  Crustacea  and  Insects. 
A  part  of  the  dorsal  vessel  is  much  enlarged  and  very  muscular. 
By  its  contraction  the  blood  is  forced  forward  and  backward 
through  branching  vessels  to  all  parts  of  the  body.     On  its 
return  the  blood  enters  large  spaces,  which  represent  the  body 
cavity  and  thus  it  reaches  a  space  immediately  surrounding 
the  heart,  the  pericardial  cavity.     The  heart  is  pierced  by  six 
or  eight  pairs  of  openings  guarded  by  valves.     When  the  heart 
expands  the  blood  enters  by  these  openings  (ostia),  but  when  it 
contracts  the  closing  of  the  valves  prevents  the  return  of  the 
blood  through  the  ostia.     It  is  therefore  forced  out  through  the 
vessels.     (See  Fig.  no.) 

472.  In  Vertebrates  the  circulatory  system  is  always  closed 
and  the  heart  is  developed  into  a  powerful  pumping  organ  with 
two,  three  or  four  chambers.     From  the  intestine  the  absorbed 
food  is  first  carried  to  special  organs  in  which  it  undergoes 


CIRCULATION 


207 


FIG.  114.— Diagram  to  show  the  general  plan  of  the  circulation  in  mammals, 
i,  Left  ventricle;  2,  aortic  arch;  3,  dorsal  aorta;  4,  postcaval  vein;  5,  right 
auricle;  6,  right  ventricle;  7,  pulmonary  artery;  8,  pulmonary  veins  (the  pul- 
monary veins  open  into  the  left  auricle  which  in  turn  opens  into  the  left  ventricle). 
The  order  of  the  numbers  1-8  indicates  the  course  taken  by  the  blood  in  com- 
pleting a  circuit  of  the  systemic  and  pulmonary  circulations.  10,  the  thoracic 
(lymphatic)  trunk;  n,  precaval  vein.  Dig.,  The  digestive  tract;  H.P.V., 
hepatic  portal  vein;  Liv,  liver;  P,  lung. 


2O8  ANIMALS 

further  changes  before  it  is  admitted  to  the  general  circulation. 
The  carbo-hydrates  and  peptones  are  collected  by  the  hepatic 
portal  vein  and  carried  to  the  liver,  where  certain  substances 
are  absorbed  and  ultimately  pass  back  into  the  intestine.  This 
occurs  in  the  case  of  some  substances  which  would  be  deleterious 
if  permitted  to  pass  into  the  general  circulation.  Excess  car- 
bo-hydrates are  also  stored  temporarily  in  the  liver  and  other 
organs  in  the  form  of  glycogen.  The  fats  are  broken  up  into 
fatty  acids  and  glycerine,  and  then,  after  absorption,  resyn- 
thesized  as  fats  of  a  different  kind -in  the  cells  of  the  mucous 
epithelium.  They  finally  appear  as  globules  in  the  lacteal 
capillaries  of  the  villi  and  thus  come  into  the  blood  through  the 
thoracic  duct. 

473.  Fats  are  also   stored,   sometimes  in  large  quantities, 
and  represent  a  large  reserve  of  energy.     They  are  usually 
found  in  the  connective  tissues,  under  the  skin,  among  the 
muscles,  covering  the  visceral  organs  and  elsewhere.     From  the 
liver  the  absorbed  food  materials  get  into  the  circulation  through 
the  inferior  vena  cava,  while  the  lacteals  pour  their  contents 
into  the  thoracic  duct  and  thus  into  the  left  sub-clavian  vein. 

474.  So  long  as  the  nourishing  fluids  remain  in  the  blood 
vessels  they  can  be  of  no  service  to  the  tissues.     But  the  walls 
of  the  capillaries  are  so  thin  that  the  fluid  portion  of  the  blood 
can  seep  through.     In  this  way  the  lymph  arises  which  is  found 
in  all  the  living  tissues  of  the  body,  filling  the  minute  spaces 
between  the  cells.     Fresh  supplies  of  lymph  are  continually 
escaping  from  the  capillaries  and  the  impoverished  lymph  drains 
off  out  of  the  lymph  spaces  into  the  lymph  vessels,  which  finally 
empty  into  the  thoracic  duct.     Thus  the  lymph  enters  the  cir- 
culation agin. 

RESPIRATION 

475.  When  the  fuel  is  consumed  in  the  firebox,  to  return  to 
the  analogy  of  the  steam  engine,  there  must  be  free  access  of 


RESPIRATION  2OQ 

air,  specifically  the  oxygen  of  the  air.  Otherwise  the  combus- 
tion will  not  continue,  the  fire  will  die  out,  and  the  engine  fin- 
ally come  to  a  standstill.  An  animal  also,  when  deprived  of  air, 
soon  goes  into  a  quiescent  state,  and  when  active,  the  amount 
of  air  required  varies  with  the  energy  expended.  The  living 
animal  is  also  continually  evolving  COi,  and  that,  too,  in  pro- 
portion to  the  energy  expended.  It  is  evident,  therefore,  that 
there  is  combustion,  or  oxidation  of  carbon,  going  on  in  the 
organism.  It  is  known  that  this  process  takes  place  in  the 
tissues,  i.  e.,  in  the  cells,  and  we  must,  therefore,  account  for 
the  presence  of  oxygen  in  the  tissues. 


FIG.  115. — Part  of  the  body  of  nereis,  showing  the  respiratory  organs.  The 
broad  superior  ligula  of  the  dorsal  ramus  of  each  parapodium  has  a  thin  integu- 
ment and  is  richly  supplied  with  blood-vessels. 

476.  Amoeba  and  many  other  organisms  can  absorb  enough 
oxygen  through  the  general  surface  of  the  body.  Even  com- 
paratively large  animals,  because  of  their  form  and  peculiarity 
of  structure,  can  obtain  enough  oxygen  in  this  way.  The  sea- 
anemone,  for  example,  though  comparatively  large,  exposes 
not  only  the  external  surface  of  the  body  and  tentacles,  but  the 
much  larger  folded  surface  of  the  gastro-vascular  cavity  is 
exposed  to  the  water,  which  is  being  continually  renewed  by 
currents  passing  in  and  out  of  the  mouth.  Even  the  frog,  when 
14 


2IO 


ANIMALS 


quiescent,  may  have  its  demands  for  oxygen  satisfied  by  ab 
sorption.  through  the  skin.  But  the  more  compactly  buil 
animals,  even  when  not  large,  are  provided  with  special  organ: 
for  the  absorption  of  oxygen.  The  earthworm  is  among  th< 
largest  of  animals  destitute  of  such  organs.  But  the  earth  worn 
is  unable  to  absorb  enough  oxygen  when  in  water  and  wil 
ultimately  drown.  Nereis  possesses  a  pair  of  flat  plates  ii 


FIG.  116. — Cross  section  of  crawfish  in  the  thoracic  region,  a,  Appendage 
c,  carapace;  cf,  part  of  carapace  covering  the  gill  chamber;  d,  digestive  tract 
g,  gill;  h,  heart;  /,  liver;  m,  m',  muscles;  n.c.,  nerve  cord;  p.s.,  pericardial  sinus 
r,  gonad;  st,  sternal  artery;  va,  ventral  artery;  vs,  ventral  blood  sinus.  (Fron 
Galloway,  after  Lang.) 

each  segment,  one  on  each  parapodium,  which  are  richly  sup 
plied  with  capillaries  lying  very  near  the  surface.  These  supple 
ment  the  general  body  surface  in  the  absorption  of  oxygen 
Even  here  the  worm  feels  the  necessity  of  keeping  the  water  ir 
motion  in  order  to  bring  in  fresh  supplies  of  oxygen.  Wher 
the  animal  is  at  rest  the  body  keeps  up  a  rhythmical  undulating 
movement  by  which  the  water  is  kept  in  motion. 


RESPIRATION 


211 


477.  The  crayfish  bears  under  a  fold  of  the  carapace,  on  either 
side,  a  large  number  of  brush-like  gills,  composed  essentially  of 
numerous  slender  thin-walled  filaments,   through  which  the 
blood  constantly  circulates.     There  is  also  a  special  structure 
in  the  form  of  a  curved  paddle  or  spoon,  which  by  its  motion 
keeps  the  water  constantly  moving  through  the  gill  chamber. 
This  highly  efficient  set  of  organs  evidently 

makes  good  the  deficiency  in  absorbing  power 
of  the  body  surface  resulting  from  the  imper- 
vious cuticular  integument. 

478.  In  Insects,  a  unique  method  of  aerating 
the  body  has  developed.     The  air  is  carried  to 
all  parts  of  the  body  by  an  intricate  system  of 
slender  tubes,  tracheae,  which  open  on  the  sur- 
face through  small  pores  in  the  integument,  the 
stigmata.     The  air  is  forced  into  and  out  of 
these  tubes  by  a  telescoping  action  of  the  rings 
of  the  abdomen. 

479.  In  Fishes,  the  gills  are  not  unlike  those 
of  the  crayfish,  but  the  water  current  is  produced 
in  a  different  way.     The  water  is  first  taken  into 
the  pharynx  through  the  mouth,  and  from  the 
pharynx  it  passes  through  a  series  of  slits  be- 
tween the  arches   which   bear   the  gills.     A  pair  of  delicate 
membranes  at  the  mouth  serve  as  valves  and  cause  a  flow  of 
water,  always  in   the  same  direction,  to  result  from  merely 
opening  and  closing  the  mouth. 

480.  In  a  few  fishes  and  in  the  adult  stage  of  all  other  Verte- 
brates, a  pair  of  air  sacks  or  lungs  take  the  place  of  the  gills  of 
the  fish.     In  the  lower  forms  the  lungs  are  comparatively 
simple,  the  inner  surface  of  the  air  sacks  being,  at  most,  some- 
what  folded.     In    the   birds   and   mammals,    however,    they 
become  exceedingly  complex,  through  the  folding  of  the  walls  to 
increase  the  absorbing  surface.     The  lung  first  appears  as  a 


FIG.  117. — Dia- 
gram of  a  feather- 
like  gill.  This 
type  is  found  in 
the  crayfish. 


212 


ANIMALS 


pocket  in  the  ventral  wall  of  the  digestive  tract  in  the  region 
of  the  pharynx.  This  pocket  divides  into  two  branches  which 
develop  into  the  right  and  left  lungs.  Each  branch  divides 
many  times  so  that  a  very  complicated  system  of  tubes  is 
formed.  The  air  tubes  are  thin  walled  and  a  network  of  blood 
capillaries  closely  surrounds  them. 

481.  Inspiration  in  the  Amphibia  and  a  few  Reptiles  is  a 
process  analogous  to  swallowing.     But  in  most  Reptiles  and  in 


Vp 


FIG.  118. — Three  early  stages  in  the  development  of  a  mammalian  lung.  In 
B  the  alimentary  canal  is  shown  extending  upward  directly  above  the  letter  B. 
Ep,  I,  and  II,  the  bronchial  tubes.  Ap,  pulmonary  artery;  Vp,  pulmonary 
veins.  (McMurrich,  after  His.) 

Birds  and  Mammals  the  air  is  forced  into  the  lungs  by  atmos- 
pheric pressure  upon  muscular  expansion  of  the  thoracic  cavity. 
The  latter  is  brought  about  by  elevation  of  the  ribs  and,  in 
Mammals,  by  depression  of  the  dome-shaped  diaphragm. 

482.  The  course  which  the  blood  takes  may  or  may  not  have 
a  fixed  relation  to  the  respiratory  organs.  In  Worms  and 
Crustacea  some  blood  is  continually  being  oxygenated,  and  this, 
mingling  with  the  rest,  is  sufficient  for  the  needs  of  the  animal. 
In  Fishes,  all  the  blood  passing  through  the  heart  is  forced 
through  the  gills  and  then  passes  on  to  the  tissues  of  the  body. 


RESPIRATION  213 

In  Frogs  and  Reptiles  the  oxygenated  blood  coming  from  the 
lungs  and  that  coming  from  the  other  tissues  of  the  body  mingle 
to  some  extent  in  the  heart,  and  this  mixed  blood  is  then  sup- 
plied to  the  tissues  of  the  body.  In  Birds  and  Mammals  again, 
through  the  complete  separation  of  the  respiratory  and  systemic 
circulation,  all  blood  passes  alternately  through  the  lungs  and 
the  body  tissues. 

483.  Oxygen  is  taken  up  by  the  blood  as  air  is  absorbed  by 
water,  but  in  most  animals,  excepting  Insects,  there  is  a  sub- 
stance present  in  the  blood  which  has  a  special  affinity  for  oxy- 
gen.    In   some    cases,    especially   among   invertebrates,    this 
substance  forms  part  of  the  blood  plasma;  in  others,  including 
all  Vertebrates,  it  resides  in  certain  cells  floating  in  the  blood, 
the  red  blood  corpuscles.     In  either  case  it  gives  the  character- 
istic color  to  the  blood.     In  some  invertebrates,  the  earthworm, 
for  example,  and  all  Vertebrates,  the  substance  is  red  and  con- 
tains iron.     In  other  cases,  some  Crustacea  and  some  Mollusca, 
the  substance  is  blue  and  contains  copper.     The  first  is  called 
haemoglobin,   the  latter  haemocyanin.     There  are  also   some 
others,   more  rare.     These   substances  have    an  affinity    for 
oxygen,  so  that  the  blood  is  enabled  to  carry  more  oxygen  than 
it  otherwise  could.     In  passing  through  the  respiratory  organs 
the  oxygen  carriers  become  charged  with  oxygen  and  assume  a 
brighter  color.     In  passing  through  the  tissues  where  oxygen  is 
needed  the  haemoglobin,  or  haemocyanin,  again  assume  a  darker 
color  because  of  the  loss  of  oxygen  to  the  tissues.     The  red 
corpuscles  originate  in  the  red  marrow  of  the  bones.     They  are 
short  lived  and  disintegrate  in  the  liver  and  form  the  red  and 
green  pigments  of  the  bile. 

METABOLISM 

484.  In  green  plants,   the  protoplasm  takes  up  inorganic 
substances,  such  as  water,  carbon-dioxide,  nitrates  and  other 
mineral  salts  containing  sulphur,  phosphorus,  iron,  calcium, 


214  ANIMALS 

magnesium,  potassium,  and  others.  From  these  the  simpler 
organic  compounds,  such  as  the  carbohydrates,  are  formed,  also 
more  complex  nitrogenous  substances  like  aleurone  and  finally 
protoplasm  itself.  Animals,  however,  lack  the  power  of  build- 
ing up  protoplasm  from  its  inorganic  constituents.  They 
require  food  containing  organic  nitrogenous  compounds  like 
aleurone,  albumen  and  protoplasm.  These  may  be  supple- 
mented by  the  simpler  carbon  compounds,  like  the  carbo- 
hydrates, fats  and  oils.  The  nitrogenous  substances,  are 
necessary  wherever  growth  or  repair  are  taking  place,  i.  e., 
wherever  protoplasm  is  being  formed.  The  carbon  compounds 
may  be  used  as  well  as  the  nitrogenous  where  there  is  merely  an 
evolution  of  energy  demanded,  as  in  locomotion  and  the  produc- 
tion of  heat.  The  details  of  the  processes  which  take  place  in 
the  cell  are  not  known.  But  when  foods  are  assimilated, 
growth  takes  place  and  the  cell  becomes  energized  so  that  it  is 
capable  of  performing  the  functions  peculiar  to  it. 

485.  The  results  of  the  activities  of  the  cell  may  be  briefly 
summarized: 

486.  Growth  is  the  most  general  result  of  assimilation,  but 
need  not  be  further  discussed  here. 

487.  In  glandular  cells,  activity  results  in  the  formation  of 
the  special   secretions  which  are  characteristic  of  the  gland. 
These  may  be  used  in  the  building  up  of  permanent  structures 
of  the  organism,  such  as  bone  or  cartilage,  or  the  secretions 
may  have  only  a  temporary  value,  and  after  they  have  served 
their  purpose,  be  eliminated  from  the  body  as  slime  and  oil 
from  the  glands  of  the  skin.     With  this  class  may  be  included 
those  cells  which  produce  substances  by  the  transformation  of 
protoplasm,  although  in  the  true  glandular  cell  the  secretions 
have  probably  not  reached  the  complexity  of  structure  of  proto- 
plasm.    Cuticular  and  epidermal  structures  are  of  the  trans- 
formed protoplasm  type. 

488.  The  activity  of  muscle  is  manifested  primarily  by  a 


METABOLISM,   EXCRETION 


215 


contraction  and  secondarily  by  the  production  of  heat,  but  at 
the  same  time,  substances  are  formed  and  set  free,  which  show 
that  chemical  processes  are  at  work  and  which  give  a  clue  as  to 
the  nature  of  those  processes.  The  oxygen  and  food  substances, 
C6Hi2O6,  let's  say,  make  their  appearance  again,  but  in  an 
altered  form.  Carbon  dioxide  is  produced  in  large  quantities 
together  with  other  substances  which  must  be  eliminated  from 
the  body,  or  else  serious  disorders  occur.  Nitrogen  waste 
compounds  are  also  formed,  and  like  the  CO2,  they  have  a  lower 
energy  value  than  the  substances  from  which  they  were  derived. 

EXCRETION 

489.  The  waste  matters  produced  by  metabolism  are  soluble 
in  the  body  fluids  and  in  water.  Hence,  small  animals  like 
amoeba  and  hydra  can  eliminate  them  from  the  body  surface  by 


-TV 


FIG.  119. — Diagram  of  a  nephridium  of  an  Annelid,  b,  bf,  blood-vessels; 
c,  coelom;  d,  duct;  e,  opening  through  the  epidermis;  /,  funnel;  gl,  glandular 
portion;  s,  mesentery;  W,  wall  of  body;  iv,  wall  of  intestine.  (From  Galloway.) 

diffusion.  In  larger  animals  the  substances  excreted  by  the 
cells  pass  out  into  the  body  fluid  or  the  blood  current  and  are 
thus  carried  to  the  place  of  elimination.  The  CO2  being  a  gas 
is  chiefly  given  off  from  the  organs  of  respiration,  following  the 
path  of  the  oxygen,  but  in  the  reverse  direction.  The  volume  of 
oxygen  absorbed  in  the  human  lung'is  about  5  per  cent,  of  the 
inspired  air.  The  volume  of  C02  given  off  is  a  little  over  4 


2l6  ANIMALS 

per  cent,  on  the  average.  If  only  carbohydrate  foods  were 
assimilated  the  percentage  of  these  gases  should  be  equal,  but 
the  oxygen  consumed  with  hydrocarbon  and  proteid  foods  in 
part  leaves  the  body  by  another  path. 

490.  The  nitrogenous  waste  matters  are  not  gaseous  and, 
therefore,  cannot  be  eliminated  by  the  lungs,  and  in  fact,  we 
find  in  all  the  higher  animals  a  special  set  of  organs  for  this 
function.  The  organs  which  presumably  perform  this  function 


FIG.  i2o.-^A  section  through  the  nephridium  of  nereis  showing  the  funnel  in 
longitudinal  section  and  the  convoluted  tubule  cut  across  at  many  points.  The 
blood-vessels  are  also  cut  at  several  points  (black).  B.W.,  The  body  wall; 
B.V.,  blood-vessel;  c,  coiled  tubule;  F,  funnel. 

in  Worms  are  pairs  of  tubules  arranged  segmentally,  one  pair 
in  each  segment.  They  are  called  nephridia  and  consist  of 
slender,  more  or  less  coiled,  tubes  which  open  into  the  body 
cavity  by  a  ciliated  funnel-like  opening.  The  other  end  of  the 
tubule  opens  by  a  pore  on  the  surface  of  the  body. 

491.  In  the  crayfish  there  are  organs,  the  " green  glands," 
which  are  probably  homologous  to  nephridia,  but  there  is  only 
a  single  pair,  located  at  the  base  of  the  antennae.     They  are 
comparatively  large  organs  and  more  complicated  in  structure. 

492.  The  single  pair  of  kidneys  of  Vertebrates  are  much 
more  complicated  excretory  organs  and  yet  the  uriniferous 
tubules  of  the  kidney  resemble  the  nephridia  of  the  worm  and 
are  probably  homologous  organs.     The  kidney  tubule  is  a  long, 


EXCRETION,   REPRODUCTION  2IJ 

slender  convoluted  tube  which  in  the  primitive  condition  has  a 
funnel  like  the  nephridium,  but  in  the  mature  condition  of  the 
mammal  it  is  closed.  Instead,  however,  a  considerable  part  of 
its  wall  is  closely  applied  to  complex  knots  and  networks  of 
blood  capillaries  from  which  the  secreting  cells  of  the  tubule 
extract  the  nitrogen  waste  matter.  The  kidney  tubules  all 
open  into  a  common  chamber  from  which  a  duct,  the  ureter, 
leads  to  a  reservoir,  the  urinary  bladder.  The  nitrogenous 
wastes  leave  the  tissues  with  the  lymph  and  thus  are  carried 
back  into  the  general  circulation.  In  this  way  they  reach  the 
kidneys.  The  chief  waste  drawn  from  the  blood  by  the  kidneys 
is  urea,  CON2H4,  but  there  are  a  number  of  other  substances 
eliminated  in  much  smaller  volume. 

493.  The  liver  of  the  higher  animals  seems  to  have  several 
functions,  one  of  which  is  excretion.     The  bile  secreted  by  the 
liver  is  a  complex  substance  and  its  significance  is  not  fully 
understood.     Its   function   in   digestion   is   probably   only   a 
secondary  one.     It  contains  waste  matters  taken  from  the  blood 
and  these  are  eliminated  through  the  intestine. 

494.  The  white  blood  corpuscles  also  assist  in  ridding  the 
body  of  useless  or  deleterious  substances. 

REPRODUCTION 

495.  Under  favorable  conditions  an  amoeba  will  occasion- 
ally divide  into  two  similar  parts.     These  parts  then  continue 
to  grow  and  after  a  time  they  also  divide.     This  phenomenon 
is  one  of  the  characters  of  the  living  cell.     The  impulse  to  divide 
does  not  seem  to  depend  upon  any  special  external  stimulus. 
It  is  the  normal  consequence  of  growth  under  favorable  condi- 
tions.    The  process  of  division  requires  from  a  few  minutes  to 
an  hour  from  beginning  to  completion,  and  may  be  repeated 
after  a  number  of  hours  to  several  days. 

496.  With  regard  to  the  details  of  the  process  of  division  there 
are  two  types.     In  one  case  it  is  much  more  complicated  than 


2l8  ANIMALS 

in  the  other.  In  the  simpler  type  the  first  evidence  that  di- 
vision is  about  to  take  place  is  seen  in  a  slight  elongation  of  the 
nucleus.  This  proceeds  until  the  nucleus  assumes  the  shape  of 
a  dumb-bell.  The  two  halves  continue  to  draw  apart  until 
only  a  slender  strand  connects  them  and  this  finally  breaks. 
As  the  division  of  the  nucleus  proceeds  the  body  of  the  cell  also 
elongates.  The  pseudopodia  are  formed  only  at  the  two  ends. 
The  cell  becomes  constricted  in  the  equatorial  plane  and  this 
cuts  deeper  into  the  cell  until  the  latter  is  finally  cut  into 
two  approximately  equal  parts.  The  two  daughter  nuclei  have 
by  this  time  assumed  the  normal  rounded  form  and  there  are 
then  two  amoebae.  In  the  division  the  contractile  vacuole 
remains  in  one  of  the  daughter  cells,  but  before  division  is 
complete  a  new  vacuole  has  been  formed  in  the  other  one. 

497.  In  other  cases,  division  comes  about  through  a  compli- 
cated process  known  as  mitosis  or  karyokinesis.     This  process 
is  described  below.     No  significance  is  known  to  attach  to  the 
difference  in  method.     The  results  are  apparently  the  same. 

498.  In  many  Protozoa,   another  interesting  phenomenon 
has  been  observed  which  should  be  mentioned  here,  although 
it  has  not  been  observed  in  amoeba.     This  is  the  phenomenon 
of  conjugation.     Two  similar  animals  unite,  either  partially 
and  temporarily  or  else  completely,  so  as  to  form  a  single  cell. 
In  the  latter  case  the  two  nuclei  fuse  into  one.     When  the 
union  is  only  temporary  the  nuclei  of  both  cells  divide  and  a  part 
of  the  nuclear  matter  from  each  cell  is  transferred  to  the  other 
cell,  where  it  unites  with  the  nucleus  of  that  cell.     By  either  of 
these  processes  cells  are  formed  with  nuclei  composed  of  ma- 
terial  derived   in   equal   parts   from    two    individuals.     The 
details  of  this  process  will  be  discussed  more  fully  in  Part  III. 
Its  significance  will  be  better  understood  when  compared  with 
the  sexual  method  of  reproduction  of  the  metazoa. 

499.  Hydra  reproduces  by  budding  and  by  development  of 
eggs.     Budding  is  a  process  found  among  other  metazoa  as 


REPRODUCTION 


219 


well  as  among  the  Coelenterata.  The  process  is  well  exempli- 
fied by  hydra.  The  bud  which  is  eventually  to  form  a  new 
polyp  is  first  seen  as  a  slight  protuberance  of  the  lower  part  of 


FIG.  121. — Diagram  of  hydra  in  longitudinal  section.  A,  Well-developed  bud 
is  shown  on  the  right;  B,  base;  o,  ovary;  T,  testis.  (From  Korschelt  and  Heider, 
after  Aders.) 

the  wall  of  the  column.  This  is  caused  by  the  more  rapid 
growth  of  the  tissues  in  this  region  and  involves  both  ectoderm 
and  entoderm.  The  bud  grows  larger,  becomes  cylindrical, 
and  finally  a  circle  of  tentacles  forms  around  the  distal  end. 


220 


ANIMALS 


The  bud  resembles  the  parent  polyp  in  form  and  may  be  half 
as  large.  With  the  opening  of  a  mouth  in  the  centre  of  the 
circle  of  tentacles  the  animal  is  complete.  Up  to  this  time 
the  gastro- vascular  cavity  of  the  bud  has  been  in  communica- 
tion with  that  of  the  parent,  but  the  base  of  the  bud  becomes 
gradually  more  constricted  until  finally  the  bud  is  cut  off 
entirely  and  is  then  an  independent  organism.  Several  such 
buds  may  be  in  process  of  development  at  one  time  and  by  this 
means  the  number  of  individuals  rapidly  grows. 


FIG.  122. — The  egg  cell  of  hydra,  in  amoeboid 
form.     (After  Kleinenberg.) 


FIG.  123. — A  hydra  embryo. 
The  first  four  tentacles  just 
beginning  to  develop.  (After 
Kleinenberg.) 


500.  Less  frequently  another  type  of  protuberance  may  be 
observed  on  the  column  of  the  hydra.  Just  below  the  circle  of 
tentacles  may  be  found  a  conical  eminence  which  affects  only 
the  ectoderm.  Lower  down  on  the  column,  frequently  on  the 
same  individual,  a  somewhat  similar,  though  more  rounded, 
protuberance  may  be  found.  These  are  the  gonads.  The 
upper  ones  are  testes  and  in  them  are  developed  the  sperm  cells, 
which  are  very  small  and  provided  with  a  flagellum.  These  are 
produced  in  large  numbers.  The  lower  gonads  are  the  ovaries 
and  contain  finally  a  single  large  cell,  the  ovum.  Both  ova 
and  sperm  cells  are  derived  from  the  ectoderm,  but  they  recede 
from  the  surface  and  are  covered  by  the  ectodermal  epithelium 
during  the  period  of  development.  When  the  egg  is  mature  it 


REPRODUCTION 


221 


becomes  exposed  by  the  breaking  of  the  ectoderm,  but  it  still 
remains  attached  by  a  stalk.  At  this  time  the  sperm  cells  are 
liberated  from  the  testis  in  large  numbers.  They  swim  about 
in  the  water  and  by  some  means,  probably  a  chemical  stimulus 
originating  in  the  egg,  they  are  attracted  to  the  egg.  One  of 
the  sperm  cells  penetrates  the  protoplasm  and  fuses  with  the  egg 
nucleus.  This  "fertilizing"  process  initiates  the  developing 
process.  A  membrane  is  first  formed  around  the  egg  and  by 
repeated  cell  division  a  cylindrical  embryo  is  developed.  The 
membrane  then  breaks  and  the  ciliated  larva  is  set  free  at  the 


FIG.  124. — Longitudinal  section  of  small  Turbellarian,  Microstomum,  which 
multiplies  asexually  by  strobilation.  b,  Brain;  c,  ciliated  pit;  d,  planes  of 
division;  e,  eye-spot;  ent,  entoderm;  g,  intestine;  gl,  gland  cells;  m,  mouth 
(original);  mf,  mouth  of  second  zooid;  m2,  m3,  mouths  of  offspring  of  second  and 
third  orders.  The  strobila  consists  of  a  chain  of  four  nearly  completed  zooids. 
(From  Galloway). 

time  when  four  tentacles  are  just  beginning  to  develop.  After 
swimming  for  a  time  the  larva  becomes  attached  and  a  mouth 
is  formed.  From  three  to  five  more  tentacles  appear  in  the 
spaces  between  the  others  and  the  young  hydra  is  complete. 
After  maturing  a  number  of  ova  the  parent  hydra  dies. 

501.  Some  annelid  worms  also  reproduce  by  asexual  methods, 
but  among  the  higher  forms  like  nereis  and  the  earthworm  repro- 
duction is  wholly  by  the  sexual  method.  In  nereis  the  sexes 
are  distinct;  each  individual  produces  either  eggs  or  sperm,  but 
not  both.  The  reproductive  cells  are  differentiated  in  size  and 
form,  very  much  as  in  hydra.  They  are  developed  from  cells 
of  the  mesodermal  epithelium  lining  the  body  cavity  (on  the 


222  ANIMALS 

wall  of  the  digestive  tube).  When  they  are  mature  they  lie 
free  in  the  body  cavity,  from  which  they  escape  into  the  water 
by  the  breaking  of  the  body  of  the  worm.  The  sperm  and  ova 
escape  into  the  water  at  the  same  time.  After  fertilization  a 
larva  is  developed  which  has  no  resemblance  to  an  Annelid 
but  is  much  more  like  a  rotifer.  This  is  called  a  trochophore 
larva  and  is  regarded  as  indicating  relationship  between  the 
Rotifers  and  Annelids.  From  one  end  of  the  trochophore  larva 


FIG.  125. — The  ovum  of  nereis.     Photomicrograph;  greatly  magnified. 

the  body  of  the  worm  is  developed  segment  after  segment. 
The  development  of  nereis  is,  therefore,  by  metamorphosis. 

502.  The  earthworm  is  hermaphroditic,  i.  e.,  both  sexes  are 
united  in  one  individual,  and  development  is  direct. 

503.  In  the  crayfish  the  reproductive  cells  are  developed  in 
special  sack-like  organs  lying  in  the  thoracic  part  of  the  body 
cavity.     The  sexes  are  separate.     There  is   a  single   ovary, 
consisting  of  a  pair  of  lateral  lobes  connected  by  a  single  median 
lobe.     A  pair  of  ducts  lead  from  the  ovary  to  the  basal  joints 
of  the  eleventh  appendages  where  they  open  to  the  exterior. 
The  testis  of  the  male  is  similar  in  position  and  composition, 


REPRODUCTION  223 

but  the  ducts  open  at  the  bases  of  the  appendages  of  the  thir- 
teenth segment. 

504.  The  eggs  are  fertilized  at  the  moment  of  their  escape 
from  the  oviducts  and  are  then  cemented  to  the  hairs  of  the  ab- 
dominal appendages  of  the  female.     In  this  way  they  are  pro- 
tected from  other  animals;  care  is  taken  that  they  have  the 
necessary  supply  of  aerated  water  and  they  are  not  carried 
away  by  currents  of  water  toward  the  sea.     Even  after  the  young 
are  hatched  they  continue  to  cling  for  some  time  to  the  appen- 
dages of  the  female. 

505.  It  happens  that  development  is  direct  in  the  case  of 
the  crayfish,  though  in  many  Crustacea  there  is  a  well  marked 
metamorphosis.     In  some  Insects  development  is  also  direct, 
as,  e.  g.,  in  the  grasshopper,  but  in  several  orders  of  Insects 
there  is  a  complete  metamorphosis.     From  the  egg  of  the  butter- 
fly is  hatched  a  small  caterpillar.     This  grows  into  a  large  cater- 
pillar.    Then  a  metamorphosis  occurs.     The  caterpillar  be- 
comes a  quiescent  pupa  and  remains  such  for  a  time;  then  an- 
other change  gives  birth  to  the  imago  butterfly. 

506.  A  few  fishes  are  hermaphroditic.     In  all  other  Verte- 
brates the  sexes  are  distinct.     The  gonads  are  developed  from 
the  mesoderm.     They  have  no  ducts  primarily,  but  certain 
tubules  which  belong  primarily  to  the  excretory  system  become 
specially  modified  and  assume  the  function  of  genital  ducts. 
In  some  aquatic  Vertebrates  the  eggs  are  fertilized  in  the  water, 
but  in  all  reptiles,  birds  and  mammals  fertilization  takes  place 
in  the  oviduct  and  development  begins  before  the  escape  of  the 
egg  from  the  body  of  the  parent.     In  the  highest  Mammals 
this  intra-uterine  development  continues  for  weeks,  months  or 
even,  in  the  case  of  the  elephant,  to  nearly  two  years.     In  some 
fishes,  and  especially  in  frogs  and  toads,  there  is  a  marked  meta- 
morphosis, but  in  the  higher  groups  the  development  is  direct. 


APPENDIX  TO  PART  II 

CLASSIFICATION  OF  ANIMALS 

507.  PHYLUM  I.     Protozoa. — Protozoa  are  found  in  nature 
practically  everywhere  where  there  is  moisture;  in  the  soil,  in 
fresh  and  salt  waters  and  even,  as  parasites,  in  the  tissues  of 
higher  animals  and  plants.     They  may  even  be  found  in  prac- 
tically dry  situations,  as  in  dust,  but  then  only  in  a  resting  or 
spore  condition.     When  Protozoa  in  this  state  are  moistened 
they  absorb  water,  the  protoplasm  swells,  the  enclosing  mem- 
brane is  broken  and  the  organism  resumes  an  active  existence. 
This  is  why  they  always  appear  when  a  little  dry  soil,  a  few  dry 
leaves  or  any  other  organic  matter  is  placed  in  a  dish  of  water. 
Many  species  are  found  the  whole  world  over,  others  are  more 
limited  in  distribution.     For  example,  some  of  the  parasitic 
forms  are  limited  to  one,  or  a  few  related  species  of  host  and 
consequently  are  limited  to  the  range  of  the  host.     Some  groups 
are  peculiar  to  fresh  waters  while  others  are  marine. 

508.  The  number  of  species  of  Protozoa  is  very  great  and 
there  is  great  diversity  in  size,  form  and  habits.     Many  are 
easily  visible  to  the  unaided  eye.     Many  others  approach  the 
limit  of  visibility  but  most  can  only  be  seen  with  the  aid  of  the 
microscope.     The  phylum  is  very  difficult  to  classify  but  most 
forms  can  readily  be  placed  in  one  or  the  other  of  the  following 
five  classes,  viz.,  Rhizopoda,  Mastigophora,  Sporozoa,  Ciliata 
and  Suctoria. 

509.  Class  I.    Rhizopoda. — This  class  is  characterized  by 
the  temporary  root-like  processes  of  the  naked  protoplasmic 
body,  by  which  locomotion  is  effected  and  food  ingested.     A 
common  example  is  amoeba. 

224 


PROTOZOA 


225 


510.  In  the  order  Am&bina,  to  which  amoeba  belongs,  there  is  no  fixed 
form  of  body;  there  is  no  membrane,  shell  or  skeleton  of  any  kind.  Repre- 
sentatives of  this  order  are  found  in  both  fresh  and  salt  waters  and  many 
are  parasitic.  Entamoeba  coli  is  a  common  harmless  parasite  in  the 
human  intestine  and  Entamoeba  histolytica  is  the  cause  of  tropical 
dysentery.  The  order  Heliozoa  comprises  rhizopods  which  have  a  spher- 
ical central  body  from  which  radiate  numerous  long,  slender,  ray-like 
pseudopodia.  The  body  may  be  either  naked  or  surrounded  by  a  gel- 
atinous or  silicious  capsule  perforated  by  numerous  pores  through  which 
the  pseudopodia  project.  Sometimes  the  cell  is  attached  to  other  objects 


FIG.  126. — Actinomma,  a  Radiolarian.  A,  Whole  animal  with  a  portion  of 
two  shells  removed  to  show  the  interior.  B,  section,  showing  concentric  shells, 
radial  spines  and  central  capsule  (c) ;  n,  nucleus;  p,  protoplasm.  (From  Galloway, 
after  Parker  and  Haswell.) 

by  a  slender  stalk.  Heliozoa  are  found  in  fresh  and  salt  water.  They  are 
never  parasitic.  The  order  Foramenifera  includes  fresh-  and  salt-water 
rhizopods  which  have  a  shell  composed  of  gelatinous  or  horny  matter,  to 
which  may  be  added  calcareous  or  silicious  secretions  deposited  by  the 
protoplasm,  or  minute  foreign  particles  like  grains  of  sand.  The  pseudo- 
podia may  be  amceboid  in  form  or  long  and  slender  like  those  of  the  heliozoa 
but  differing  from  the  latter  in  the  less  regular  arrangement  and  constantly 
changing  form.  Many  species  add  successively  larger  chambers  to  the 
first  shell.  These  are  often  in  a  spiral  arrangement.  In  the  order 
Radiolaria  there  is  found  a  peculiar  structure  called  the  central  capsule 
which  encloses  the  nucleus  and  the  central  part  of  the  protoplasm.  Out- 


226  CLASSIFICATION   OF   ANIMALS 

side  the  capsule  is  another  layer  of  protoplasm  which  contains  large 
quantities  of  a  gelatinous  secretion.  Besides  the  central  capsule  there  is  a 
skeleton,  usually  of  silica,  composed  of  radial  spines  and  concentric  shells. 
This  skeleton  is  often  of  a  very  intricate  and  beautiful  design.  The  pseu- 
dopodia  are  slender  and  branching  and  are  sometimes  supported  by  a 
slender  axial  filament.  The  Radiolaria  are  marine. 

511.  Class  II.  Mastigophora. — The  Mastigophora  are  dis- 
tinguished by  the  flagellum,  a  whip-like  vibratory  appendage 
by  which  locomotion  is  effected.     There  may  be  two  or  four  or 
even  a  circlet  of  these  flagella  but  more  often  there  is  only  one. 
Flagella  also  occur  in  other  groups  of  animals  and  also  in  plants 
but  only  as  temporary  structures.     In  the  Mastigophora  they 
are  always  present  during  the  active  life  period  of  the  organism. 
The  body  has  usually  a  definite  form  though  it  is  often  capable 
of  great  contortion.     There  is  a  nucleus   and  a  contractile 
vacuole.     The  class  may  be  divided  into  three  sub-classes,  Flag- 
ellata,  Dinoflagellata  and  Cystoflagellata. 

512.  The  Flagellata  are  widely  distributed  and  there  is  extreme  diversity 
of  form  and  habit  so  that  the  group  is  difficult  to  characterize  and  classify. 
Many  contain  chlorophyll  and  are  holophytic,  some  contain  chlorophyll 
but  also  ingest  particles  of  food.     Some  are  holozoic,  some  saprozoic  and 
many  parasitic.     Special  examples  of  the  latter  are  described  in  Part  III. 
Those  forms  which  ingest  solid  food  may  do  so  either  through  a  definite 
oral]opening  or  the  food  may  be  engulfed  at  the  surface  where  no  preformed 
mouth  occurs.     In  one  group,  the  Choanoflagellata,  the  base  of  the  single 
flagellum  is  surrounded  by  a  collar-like  membrane.     The  flagellum  jerks 
the  food  particles  against  the  outside  of  the  collar  and  from  there  they  pass 
into  the  cell-body.     Undigested  fragments  of  food  substances  are  ejected 
at  the  base  of  the  flagellum,  within  the  collar.     Some  of  the  Flagellates 
form  swimming  colonies  by  the  adhesion  of  a  group  of  cells  to  each  other. 
Others  are  stalked  and  adhere  in  groups  to  form  fixed  colonies     The 
Dinoflagellata  are  highly  specialized  fresh-water  or  marine  Mastigophora. 
The  cell  has  two  flagella,  usually  placed  at  right  angles  to  each  other,  and 
hidden  in  deep  grooves  on  the  surface  of  the  cell  wall.     The  cell  often  has 
a  very  odd  form  and  is  covered  with  cellulose  plates  which  are  fancifully 
ornamented.     Many  species  contain  chromatophores  of  yellowish,  brown- 
ish or  greenish  color.     The  Cystoflagellata  are  another  small  group  of 


PROTOZOA 


227 


Mastigophora.  They  have  one  flagellum  and  locomotion  may  be  assisted 
by  rhythmical  motions  of  the  protoplasm.  They  are  large  and  contain 
considerable  gelatinous  matter  enclosed  within  the  strong  pellicula.  One 
species,  Noctiluca  miliaris,  which  is  found  in  all  seas  is  largely  responsible 
for  the  phosphorescence  of  the  water. 


FIG.  127. — Eimeria  Schubergi,  a  sporozoan  parasitic  in  the  intestinal  epithe- 
lium of  Lithobius.  A-C,  Three  steps  in  the  formation  of  sporozoites  (asexual); 
D,  microgametes;  E,  macrogamete;  F-G,  fertilization;  H-K,  three  steps  in  the 
formation  of  spores  (sexual). 

51-3.  Class  III.  Sporozoa. — The  class  Sporozoa  includes 
those  protozoa  which  at  one  time  in  their  life  cycle  multiply 
by  the  formation  of  spores.  The  spores  are  usually  enclosed  in 


228  CLASSIFICATION   OF   ANIMALS 

a  spore  case  but  this  may  be  wanting,  as,  e.  g.,  when  there  is 
an  alternation  of  hosts.  The  number  of  spores  in  a  case  is 
usually  numerous  but  sometimes  they  are  few  or  only  one.  All 
Sporozoa  are  parasitic.  They  are  very  commonly  cell  parasites, 
either  in  the  young  stages  or  permanently.  They  absorb  fluid 
food  by  osmose.  The  Sporozoa  are  widely  distributed  and 
infect  all  groups  of  the  higher  animals,  especially  worms,  arthro- 
pods, tunica tes,  molluscs  and  vertebrates.  Most  species  are 
limited  to  one  or  a  few  host  species.  The  passage  from  one 
host  to  another  similar  (not  alternate)  host  is  effected  in  the 
spore  stage.  There  is  frequently  another  method  of  repro- 
duction which  takes  place  wholly  within  a  single  host.  This 
may  alternate  with  the  spore-producing  generation  thus  giving 
rise  to  a  regular  alternation  of  generations.  Many  Sporozoa 
are  comparatively  harmless  parasites  but  among  them  are  also 
some  of  the  most  dangerous.  Several  examples  are  described 
under  the  head  of  parasitism. 

514.  Class  IV.  Ciliata. — The  Ciliata  all  possess  as  loco- 
motor  organs,  numerous  minute  vibratile  processes  called  cilia. 
They  are  widely  distributed.  Very  few  are  parasitic,  some  are 
saprozoic,  but  most  are  holozoic.  A  few  even  are  carnivorous. 
They  are  generally  free-swimming  but  some  attach  themselves 
temporarily  to  other  objects  and  some  are  permanently  fixed 
by  a  stalk.  The  cilia  serve  for  driving  currents  of  water 
containing  food  particles  to  the  mouth  and  in  the  fixed  forms 
this  is  the  chief  function  of  the  cilia.  The  form  of  the  body  is 
definite  but  the  animal  often  has  the  power  of  considerably 
changing  the  form  by  contraction.  The  cell  is  covered  by  a 
dense  protoplasmic  layer  called  a  pellicula.  There  are  usually 
two  nuclei,  a  large  macronucleus  and  a  small  micronucleus. 
Multiplication  is  effected  by  division  or  by  budding.  Some- 
times this  is  accompanied  by  the  formation  of  a  protecting  mem- 
brane or  cyst.  Cysts  are  also  formed  when  conditions  are  un- 
favorable and  represent  a  resistant  condition. 


PROTOZOA  229 

515.  The  Ciliata  are  classified  on  the  basis  of  the  arrangement  of  the 
cilia.  The  Holotricha  have  no  special  zone  of  cilia  in  the  region  of  the 
mouth.  The  Heterotricha  have  a  left-hand  spiral  of  larger  cilia  around  the 
mouth.  The  Oligotricha  have  a  spiral  or  circle  of  cilia  around  the  peri- 
stome  which  is  anterior  and  at  right  angles  to  the  axis  of  the  body.  Else- 
where the  body  is  almost  or  wholly  destitute  of  cilia.  The  Eypotricha  are 
flattened  dorso-ventrally.  The  adoral  spiral  is  on  the  ventral  side  and  the 
dorsal  side  is  without  motile  cilia.  The  Peritricha  have  the  adoral  spiral 
right-handed,  otherwise  the  body  is  not  ciliated,  many  are  stalked  and 
colonial. 


FIG.  128. — Paramcecium.  A,  Anterior;  c,  cilia;  e.c.,  ectoplasm;  e.n.,  endo- 
plasm;  f.v.,  food  vacuole;  g,  gullet;  N,  macronucleus;  n,  micro  nucleus;  o,  oral 
groove;  p.v.,  contractile  vacuole;  tr,  trichocysts;  v,  food  vacuole. 

516.  Class  V.     Suctoria. — In  this  group  there  are  no  organs 
of  locomotion  in  the  adult  and  consequently  all  are  sessile  or 
at  least  motionless.     They  are  provided  with  long  tubular  pro- 
cesses by  which  they  catch  their  prey.     Through  these  tubes 
they  then  suck  the  protoplasm  of  the  small  animals  they  have 
caught.     The    young    are    formed    by    budding.     They    are 
provided  with  cilia  by  which  they  swim  about  for  a  time  before 
becoming  attached.     Some  suctoria  are  parasitic.     The  group 
is  not  large  and  is  comparatively  unimportant. 

Metazoa 

517.  The  Metazoa  are  multicellular  animals.     In  the  embryo  the  cells 
are  arranged  in  three  distinct  layers,  an  outer  ectoderm,  an  kiner  entoderm 


230 


CLASSIFICATION   OF   ANIMALS 


and  a  middle  mesoderm.  In  the  coelenterates  the  mesoderm  is  only 
incompletely  developed  and  in  some  cases  entirely  wanting.  Reproduc- 
tion is  generally  of  the  sexual  type  though  in  the  lower  phyla  asexual 
methods  often  occur  in  addition  to  the  sexual. 

518.  PHYLUM  II.  Calenterata. — In  the  Coelenterates  the 
mesoderm  is  represented  usually  by  a  gelatinous  matrix  con- 
taining various  cellular  elements  which  are  derived  either  from 
the  ectoderm  or  entoderm.  There  is  no  body  cavity  or  vas- 
cular system.  The  gastric  cavity  is  extended  by  canal-like 
prolongations  into  all  parts  of  the  body. 


FIG.  129. — Diagram  of  a  sponge,  c,  Cloaca;  ch,  flagellale  chambers;  sp,  in- 
current  pores;  ip,  excurrent  pores;  mes,  mesenchyme;  o,  osculum;  r,c.,  radial 
canals.  (From  Galloway.) 

519.  Class  I.  Porifera. — Sponges  are  all  marine  with  the 
exception  of  a  single  fresh-water  genus,  Spongilla.  They  are 
always  attached  to  some  object  and  sometimes  bore  into  shells 
or  calcareous  rocks.  They  vary  greatly  in  form;  sometimes 
covering  the  substratum  like  a  thin  velvety  crust,  sometimes 
rising  into  conical,  spherical,  cylindrical  or  vase-shaped  masses. 


PORIFERA  23 1 

They  are  often  branched,  especially  the  cylindrical  ones  and 
the  more  massive  forms  may  become  very  irregular  in  shape 
through  the  development  of  new  parts  by  irregular  budding. 
Some  are  very  delicate  and  fragile  while  others  are  very  firm, 
even  stony.  The  color  is  as  variable  as  the  form;  they  are  often 


FIG.  130. — A  large  cup-shaped  sponge  (Poterion?)  from  the  Philippine  Islands. 

X  1/8. 

a  dull  gray  but  highly  colored  species  are  very  common.  Or- 
ange, sulphur  yellow,  violet,  purple  and  green  sponges  often 
give  color  variety  to  the  sea  bottom.  The  fresh- water  Spongilla 
is  usually  green;  the  color  in  this  case  being  due  to  the  presence 


232 


CLASSIFICATION   OF   ANIMALS 


of  minute  algae  imbedded  in  the  tissue  of  the  sponge  in  a  sym- 
biotic relationship.  In  size  sponges  vary  from  a  fraction  of  an 
inch  to  several  feet  in  diameter. 


FIG.   131.— Stylo tella  heliophila,  a  typical  sponge.     Beaufort  Harbor,  N.   C. 

X    1/2. 

520.  A  sponge  is  essentially  a  tubular  structure,  the  walls  of 
which  consist  of  three  layers,  an  outer  thin  epithelium,  the 
ectoderm,  an  inner  epithelium  of  collared  flagellate  cells,  the 


PORIFERA  233 

entoderm,  and  a  middle  gelatinous  connective- tissue  matrix 
in  which  are  embedded  branched  connective-tissue  cells, 
calcareous  or  silicious  spicules,  primitive  muscle  cells  and  repro- 
ductive cells.  Through  the  walls  of  the  sponge  numerous  fine 
pores  or  slender  canals  penetrate  from  the  exterior  to  the 
central  larger  canal  or  cloaca.  In  the  simpler  sponges  the 


FIG.  132. — A  Niaxon  sponge  (Pheronema?).     Philippine  Islands.     X  1/2. 

surface  of  the  cloaca  is  lined  with  the  collared  flagellate  cells. 
These  have  a  marked  resemblance  to  the  protozoan  Choano- 
flagellata  and  are  not  found  in  any  other  metazoa.  The 
lashing  of  the  flagella  creates  a  current  in  the  water  which  flows 
inward  at  the  pores  and  outward  at  the  osculum,  the  large  open- 
ing of  the  cloaca.  The  flagella  and  collars  together  serve  for 


234  CLASSIFICATION   OF   ANIMALS 

the  capture  of  food  particles  as  in  the  Choanoflagellata  and 
digestion  is  likewise  intra-cellular. 

521.  In  more  complex  sponges  the  flagellate  epithelium  is 
limited  to  certain  depressions  of  the  cloacal  surface  which  form 
chambers  radiating  from  the  cloacal  cavity.     These  are  called 
flagellate    chambers    or    radial    canals.     In    many    cases    the 
flagellate  chambers  are  so  far  removed  from  the  cloaca  that 
another  system  of  canals  results,  the  excurrent  canals,  which 
connects   the  flagellate   chambers   with   the   cloaca.     In  still 
more  complex  forms  the  incurrent  and  excurrent  canals  are 
branched. 

522.  Sponges  have  no  power  of  locomotion  and  only  in  some 
cases  can  any  evidence  of  contraction  be  observed  directly. 
However,  the  minute  pores  can  be  closed  by  the  contraction  of 
the  muscle  cells  of  the  mesoglea.     Some  sponges  are  quite  soft, 
almost  jelly  like,  but  usually  the  mesoglea  is  so  filled  with  cal- 
careous or  silicious  spicules  as  to  render  the  sponge  firm  or  even 
hard.     In  some  sponges  the  mesoglea  contains  a  skeletal  struc- 
ture composed  of  horny  fibres,  in  addition  to  the  spicules. 
This  is  notably  the  case  with  the  common  bath  sponge  in  which 
the  spicules  are  not  well  developed. 

523.  Sponges  reproduce  by  a   process  similar  to  budding. 
A  fragment  of  the  sponge  separates,  is  carried  away  by  water 
currents,  becomes  attached  and  develops  into  a  new  sponge. 
The  sexual  method  is,  however,  the  more  frequent.     The  eggs 
and  sperm  are  developed  in  the  mesoglea,  where  the  egg  be- 
comes fertilized  and  begins  its  development.     It  escapes  as  a 
ciliated  larva,   swims   away  and   becomes  attached  with  the 
gastrula    mouth    down.     The    gastrula    cavity   becomes    the 
cloaca  and  an  osculum  is  formed  by  thinning  of  the  wall  at  the 
end  opposite  the  point  of  attachment.     The  incurrent  pores 
are  formed  in  like  manner. 

Order  i. — The  Calcispongiae  have  calcareous  spicules  of  one,  three  or 
four  rays.     Grantia. 


PORIFERA,   HYDROZOA  235 

Order  2. — The  Triaxonia  are  sponges  with  large  flagellate  chambers,  a 
thin  mesenchyme  layer  and  triaxial  silicious  spicules.  The  latter  may  be 
replaced  by  horny  fibres  or,  occasionally,  skeletal  structures  are  wanting. 

Order  3. — The  Tetraxonia  have  a  complicated  system  of  canals,  small 
flagellate  chambers  and  a  thick  mesenchyme  layer.  The  skeleton  con- 
sists of  tetraxial  or  monaxial  silicious  spicules  sometimes  combined  with, 
or  replaced  by,  spongin  fibres.  Euspongia  is  the  commercial  sponge  and 
Spongilla  the  fresh  water  sponge. 

524.  The  Cnidaria. — A  great  many  Ccelenterates  are  characterized  by 
the  possession  of  peculiar  organs,  the  cnidoblasts  or  nettling  cells.     These 
occur  in  both  ectoderm  and  entoderm  but  are  often  aggregated  in  certain 
regions,  as  on  the  tentacles.     The  nettling  cell  contains  a  small  capsule 
which  is  filled  with  a  fluid  and  contains  a  spirally  wound  thread.     A  sen- 
sory point  projects  at  the  surface.     When  this  is  stimulated  the  capsule 
bursts,  the  nettling  thread  is  turned  inside  out  and  with  it  the  fluid  content 
of  the  capsule  is  also  ejected.     The  effect  of  this  discharge  is  to  paralyze 
or  kill  the  prey.     Even  the  human  skin  is  strongly  irritated  by  the  nettling 
discharge  of  the  larger  Cnidaria  and  from  this  fact  arose  the  name.     The 
Cnidaria  are  the  Hydrozoa,  Scyphozoa  and  Anthozoa. 

525.  Class  II.    Hydrozoa. — This  class  is  named  after  the 
genus  Hydra  which  is  found  in  fresh  waters  very  widely  dis- 
tributed.    Practically  all  other  Hydrozoa  are  marine.     The 
individual  animals  of  this  class  are  always  small  but  many 
species  are  colonial  and  the  colony  may  attain  to  considerable 
dimensions.     There  is  usually  a  remarkable  alternation  of  gen- 
erations in  which  an  asexual,  fixed  polyp  form  alternates  with  a 
sexual  free-swimming  medusa.     The  polyp  is  in  most  essential 
features  like  the  hydra  in  form  but  the  lower  part  of  the  column 
is  much  elongated  and  slender  thus  forming  a  stalk.     When 
budding  occurs  the  buds  are  not  set  free  but  remain  attached 
to  the  parent  stem  and  thus  is  formed  a  colony.     At  certain 
times  another  type  of  bud  is  formed.     It  differs  in  form  from 
the  parent  polyp  and  is  usually  set  free.     This  is  the  medusa. 
Its  principal  axis  is  shorter  than  the  radial  axes  so  that  it 
assumes  the  form  of  a  saucer  or  bell  with  a  fringe  of  tentacles 
around  the  edge.     Near  the  margin  of  the  bell  on  its  concave 


236  CLASSIFICATION   OF   ANIMALS 

surface  a  thin  fold  of  the  ectoderm  projects  inward  toward  the 
principal  axis,  forming  a  circular  shelf.  This  is  the  velum, 
which  serves  to  distinguish  the  medusae  of  this  group  from  the 
Scyphomedusae. 

526.  From  the  gastric  cavity  four  radial  canals  run  out  to 
the  margin  of  the  bell  and  there  join  a  canal  which  runs  cir- 
cularly along  its  edge.     In  some  cases  there  are  six,  eight  or 
more  radial  canals.     The  gonads  are  developed  from  the  ecto- 
derm somewhere  along  the  course  of  the  radial  canals  or  on  the 
manubrium.     Special  sense  organs,  eye  spots  or  statocysts,  may 
occur  along  the  edge  of  the  bell  and  there  is  also  a  ring  of  nerve 
fibres.     The  animal  has  a  feeble  power  of  locomotion,  effected 
by  a  rhythmical  contraction  of  the  edge  of  the  bell.     The  hydro- 
medusae  vary  in  size  from  a  small  fraction  of  an  inch  to  two 
inches.     In  one  group  they  are  somewhat  larger.     Although  so 
different  in  appearance  the  medusae  are  in  reality  of  essentially 
the  same  structure  as  the  polyps.     They  are  a  little  more  highly 
developed  in  accordance  with  the  free  life  habit. 

527.  When  the  eggs  are  fertilized  a  free-swimming  ciliated 
larva  is  produced.     This  becomes  attached  by  the  aboral  pole, 
develops  tentacles  and  thus  forms  a  polyp  and  by  budding  of 
the  polyp  a  colony  is  developed.     There  is  frequently  more 
than  one  kind  of  polyp  found  in  a  colony.     In  this  case  there 
is  a  division  of  function  so  that  there  may  be  feeding  polyps 
which  are  of  the  typical  form;  protective  polyps,  without  ten- 
tacles and  mouth  but  well  supplied  with  nettling  cells;  repro- 
ductive polyps  which  produce  the  medusa  buds  for  the  colony. 
Sometimes  the  medusae  remain  attached  to  the  parent  colony 
and  there  mature  the  reproductive  cells.     In  this  case  the  me- 
dusa  is  more  or  less  rudimentary.     Such  medusae  are  then 
another  type  of  polyp  and  the  alternation    of   generations 
resolves  itself  into  a  special  case  of  polymorphism. 

Order  i. — The  Hydroidea  comprise  solitary  polyp  forms  which  have  no 
medusa  stage,  like  Hydra;  the  Trachymedusae  which  have  no  polyp  stage; 


HYDROZOA  237 

the  colonial  millepore  corals;  and  the  colonial  campanularian  and  tubu- 
larian  hydroids.  The  colonial  forms  are  all  fixed  and  usually  have  a  free 
medusa  stage  with  alternation  of  generations. 

Order  2. — The  Siphonophora  are  swimming  colonies  in  which  there  is  a 
highly  developed  stock  polymorphism  in  which  certain  individuals  form 
floats  or  swimming  bells.  The  sexual  generation  is  either  a  free  medusa 
or  an  attached  medusoid  bud.  Physalia,  the  "Portuguese  Man  of  War," 
is  a  well-known  example. 


FIG.  133. — A  hydroid  colony.     X  i. 

528.  Class  HI.  Scyphozoa. — The  Scyphozoa  are  the  com- 
mon jellyfishes.  They  are  usually  larger  than  the  medusae 
of  the  Hydrozoa,  varying  from  four  inches  to  a  yard  in  diameter. 
The  bell  is  usually  strongly  convex  and  is  made  up  of  four  anti- 
meres  though  most  of  the  organs  occur  in  multiples  of  four. 
The  margin  of  the  bell  is  lobed,  with  as  many  sense  organs 


CLASSIFICATION   OF   ANIMALS 


(tentacles,  eyespots,  statocysts)  and  nerve  centres  alternating 
with  the  lobes.  There  is  no  velum.  The  margin  of  the  peri- 
stome  is  four  cornered  with  the  four  corners  prolonged  into  "oral 

lobes"  or  branching  tentacular  arms. 
There  are  four  groups  of  gastric  fila- 
ments projecting  into  the  central  gastric 
pouch  and  the  radial  canals  branch 
many  times.  The  gonads  are  four  in 
number  and  are  developed  from  the 
entoderm  of  the  gastric  cavity  into 
which  they  project.  The  embryo  is  set 
free  as  a  ciliated  larva.  It  swims  about 
for  a  time  then  becomes  attached  and 
develops  into  a  hydra-like  polyp.  This 
polyp  is  called  a  scyphostoma  and  from 
it  many  medusae  are  formed  by  strobil- 
lation.  The  polyp  becomes  constricted 
just  below  the  circle  of  tentacles  and 
another  circle  of  tentacles  begins  to 
form  below  this  constriction.  Then  a 
third  circle  of  tentacles  and  a  third  con- 
striction begin.  This  process  continues 
downward  while  the  polyp  grows  in 
length.  The  uppermost  constriction 
grows  continually  deeper  until  a  disc- 
shaped  portion  of  the  parent  polyp 
with  a  margin  of  tentacles  is  completely 
cut  off.  There  is  thus  formed  a  minute 
medusa  which  is  called  an  ephyra. 

Later  another  disc  is  cut  off  and  then  another.  By  this  method 
a  number  of  ephyrae  are  produced  and  they  develop  directly 
into  medusae.  Sometimes  the  scyphostoma  stage  is  omitted, 
the  ciliated  planula  larva  developing  directly  into  the 
medusa. 


FIG.  134.— Physalia,  the 
Portuguese  man-of-war. 
(From  Galloway,  after 
Agassiz.) 


SCYPHOZOA 


239 


Order  i. — The  Stauro medusae  are  a  small  group  in  which  the  sense 
organs  are  wanting.  There  is  usually  an  aboral  stalk  for  attachment. 

Order  2. — The  Lobomedusae  are  free  swimming  medusae  with  sense 
organs  (tentaculocysts)  in  the  notches  between  the  lobes  of  the  umbrella. 


FIG.  135. — A  small  sea  fan  (Gorgonia)  and  a  group  of  finger  sponges.     X  1/3. 


529.  Class  IV.  Anthozoa. — The  Anthozoa  comprise  the  sea 
anemones  and  corals.  In  this  group  the  medusa  stage  is  want- 
ing and  the  polyps  are  either  solitary  and  comparatively  large 
or  smaller  and  colonial.  They  are  especially  abundant'in  the 


240  CLASSIFICATION   OF   ANIMALS 

warmer  seas  where  they  often  occur  in  vast  numbers.  Some 
forms  are  widely  distributed.  To  the  Anthozoa,  sponges  and 
sea  weeds,  is  chiefly  due  the  brilliant  coloration  so  often  found 
in  the  sea  bottom  of  tropical  and  sub-tropical  seas.  Most 
anemones  are  attached  to  some  firm  object  such  as  rocks,  sea 


FIG.  136.— A  whip  coral  (Gorgoniidse).     X  1/2. 

weeds,  shells  or  even  the  surface  of  other  animals.  This  attach- 
ment is  effected  by  a  sucking  action  of  the  basal  disc  and  permits 
the  animal  to  move  by  a  slow  creeping  motion.  Some  anemones 
lie  patfly  embedded  in  the  sand  into  which  they  can  completely 
withdraw  by  longitudinal  contraction.  Others  form  a  leathery 


ANTHOZOA 


.241 


tube  by  a  secretion  of  the  column,  still  others  secrete  calcare- 
ous matter,  especially  from  the  surface  of  the  basal  disc.  This 
is  notably  the  case  with  the  corals.  Through  this  secretion 
the  animal  becomes  immovably  fixed.  The  basal  disc  of  the 


FIG.  137. — A  branching  madrepore  coral,  Astrangia.     Slightly  reduced. 

coral  polyp  secretes  more  rapidly  along  its  edge  so  that  a  cup 
is  formed  into  which  the  animal  can  more  or  less  completely 
withdraw.  Within  this  cup  there  are  also  vertical  plates  and 
pillars  built  up  by  the  unequal  secretion  of  the  various  parts 
of  the  base.  To  this  is  due  the  beautiful  structure  of  many 
16 


24? 


CLASSIFICATION   OF  ANIMALS 


kinds  of  coral.     Some  corals  secrete  a  horny  skeleton  and  in 

some  there  is  a  mixture  of  the  horny  and  calcareous  substances. 

530.  The  larger  anemones  are  considerably  more  complex 


FIG.  138.— Coral  colonies  developing  on  a  shell.     Various  steps  in 
the  process  are  shown. 

than  the  smaller  corals  but  certain  important  characters  are 
common  to  all  and  serve  to  distinguish  this  type  of  polyp  from 
that  of  the  Hydrozoa.  The  Anthozoan  polyp  is  distinguished 
by  the  oesophagus  and  mesenteries.  The  mouth  does  not  open 


ANTHOZOA  243 

directly  into  the  gastro-vascular  cavity  as  it  does  in  the  Hydro- 
zoan  polyp.  There  is  a  long  oesophagus  which  extends  from 
the  edge  of  the  mouth  to  the  centre  of  the  gastric  cavity.  It 
is  in  reality  a  cylindrical  continuation  of  the  oral  surface  and  is 
lined  with  ectoderm.  The  gastric  cavity  is  incompletely 
divided  into  chambers  by  folds  of  the  entoderm  supported  by 
layers  of  mesoglea.  Some  of  these  mesenterial  folds  extend 
from  the  wall  of  the  column  to  the  cesophagus.  These  are  said 
to  be  complete.  Others  do  not  reach  the  cesophagus  and  are 
therefore  known  as  incomplete  mesenteries.  On  the  free  edge 
of  some  of  the  mesenteries  there  is  a  thick  muscular  cord,  the 
mesenterial  filament,  which  is  richly  supplied  with  gland  and 
nettling  cells.  There  are  no  special  sense  organs  and  nothing 
that  can  be  called  a  central  nervous  system,  though  beneath  the 
ectoderm  of  the  oral  disc  the  network  of  nerve  fibres  is  better 
developed  than  elsewhere.  A  strong  circular  muscle  is  usually 
found  just  below  the  edge  of  the  oral  disc  and  in  the  mesen- 
teries there  are  strong  longitudinal  muscle  bands.  The  gonads 
lie  embedded  in  the  mesoglea  of  the  mesenteries  along  the  free 
border.  The  sexes  are  usually  distinct.  The  larva  develops 
for  a  time  within  the  body  of  the  parent  and  escapes  as  a  ciliated 
planula  which  becomes  fixed  and  develops  directly  into  the 
polyp. 

Order  2. — The  Octactiniaria  are  chiefly  colonial.  The  polyp  has  eight 
pinnately  branched  tentacles  and  eight  mesenteries.  There  is  frequently 
a  skeletal  structure  of  horny  or  calcareous  matter.  The  whip  corals,  sea 
fans,  organ  pipe  corals,  etc. 

Order  3. — The  Ceriantipatharia  include  the  anemone  Cerianthus  and 
some  colonial  forms  with  a  horny  skeleton  and  polyps  with  six  tentacles. 

Order  4. — The  Zoanthactiniaria  comprise  the  sea  anemones  and  the 
madrepore  corals.  The  mesenteries  are  grouped  in  pairs. 

531.  Class  V.  Ctenophora. — The  Ctenophora  are  another 
group  of  jelly  fishes.  They  are  more  transparent  and  watery 
then  the  medusae  and  exceedingly  fragile.  A  common  type  is 


244  CLASSIFICATION   OF   ANIMALS 

the  pear-shaped  Pleurobrachia  which  is  also  comparable  to  a 
pear  in  size.  The  mouth  is  located  at  the  small  end  and 
opposite  it  there  lies  a  single  statocyst.  Extending  about  two- 
thirds  of  the  distance  from  pole  to  pole  and  at  about  equal 
distances  from  the  two  poles  are  eight  bands  of  vibratile  plates 
which  are  regarded  as  rows  of  cilia  fused  together.  These  are 
the  locomotor  organs.  In  place  of  nettling  cells  the  two 
long  tentacles  are  covered  with  adhesive  cells  to  which  the 
prey  adheres. 

532.  The  Ccelomata.  —  In  none  of  the  Coelenterates  are  the  fundamental 
animal  characteristics  strongly  developed.  Sense  organs  and  the  organs 
of  locomotion  are  in  no  case  highly  developed  and  the  symmetry  of  the 
body  is  always  primarily  radial  though  in  some  cases  a  tendency  toward 
secondary  bilateral  symmetry  may  be  observed.  In  the  Ccelomata  there 
is  always  a  well-developed  mesoderm.  This  makes  possible  a  more  highly 
developed  muscular  system  and  consequent  greater  locomotor  activity. 
With  this  go  also  more  highly  differentiated  sense  organs  and  bilateral 
symmetry.  The  mesoderm  is  derived  from  the  entoderm  and  encloses  a 
paired  series  of  cavities,  the  ccelomic,  or  body,  cavity. 


533-  PHYLUM  III.  Scolecida.  —  Several  classes  of  animals 
more  or  less  resembling  worms  in  the  form  of  the  body  but  with 
no  evidence  of  metameric  segmentation  are  often  called  the 
unsegmented  worms.  In  this  group  the  true  body  cavity  is 
limited  to  small  spaces  connected  with  the  excretory  and  re- 
productive organs. 

534.  Class  I.  Platyhelminthes.  —  The  animals  of  this  group 
have  a  flattened  body.  The  digestive  tract  is  sack-like,  with- 
out vent,  or  wholly  wanting.  The  space  between  the  intestine 
and  body-wall  is  filled  with  a  parenchyma  of  contractile  fibres. 
The  nervous  system  consists  of  a  paired  supra-cesophageal 
ganglion  and  a  pair  of  ventral  longitudinal  nerves.  Two  other 
pairs  of  longitudinal  nerves  are  sometimes  present.  The 
excretory  system  consists  of  a  branched  system  of  protoneph- 
ridia,  also  called  a  water  vascular  system.  The  proximal 


SCOLECIDA  245 

end  of  the  protonephridium  is  closed  by  a  so-called  flame  cell. 
This  is  a  large  cell  provided  with  long  vibrating  cilia  which 
project  into  the  proximal  end  of  the  canal.  The  flat  worms  are 
usually  hermaphrodyte  and  the  reproductive  system  is  highly 
complicated. 

535.  Order  i. — The  Turbellaria,  or  gliding  worms,  are  usually  small, 
very  much  flattened,  aquatic  animals.     The  name  refers  to -the  method 
of  locomotion  which  is  effected  through  the  cilia  by  which  the  surface  of 
the  body  is  covered.     The  mouth  is  on  the  ventral  side  and  is  usually 
provided  with  an  eversible  proboscis.     The  digestive  tract  is  a  simple 
blind  sack  in  the  smallest  microscopic  forms,  but  in  the  larger  ones  it  is 
divided  into  three  main  trunks  which  have  numerous  branches.     This 
form  of  digestive  tract  is  a  substitute  for  a  circulatory  system.     There  are 
usually  2-many  simple  eyes  at  the  anterior  end  and  over  the  brain. 

536.  Order  2. — The  Trematoda  are  parasites  and  consequently  show 
more  or  less  evidence  of  degeneration.     In  the  form  of  the  body  they 
resemble  the  Turbellaria,  but  the  surface  of  the  body  is  destitute  of  cilia 
in  the  adult.     The  animal  is  provided  with  hold-fast  organs  in  the  form 
of  hooks  and  suckers.     Commonly  there  are  two  suckers,  one  at  the 
anterior  end  enclosing  the  mouth  and  another  further  back  on  the  ventral 
side.     The  digestive  tract  is  usually  two  forked  but  may  be  more  com- 
plexly branched.     Eyes  are  only  found  in  the  ectoparasitic  forms  and 
in  some  free-living  larval  stages  of  endoparasites.     The  life  history  of  a 
trematode  is  described  in  Part  III.     Page  366. 

537.  Order  3. — The  Cestoda  are  all  endoparasites  and  in  the  adult  stage 
are  found  only  in  the  digestive  tract  of  higher  animals.     Special  sense 
organs  and  digestive  tract  are  both  entirely  wanting.     The  digested  food 
of  the  host  is  absorbed  through  the  surface  of  the  body.     In  place  of  a 
head  there  is  a  hold-fast  organ  called  a  scolex  which  in  the  most  common 
forms  has  a  circle  of  four  suckers  and  sometimes  also  a  circlet  of  hooks. 
In  a  narrower  region  just  below  the  scolex  a  process  of  strobilation  takes 
place  by  which  the  body  of  the  parasite  is  formed.     This  is  usually  com- 
posed of  a  long  series  of  proglottides,  the  ones  farthest  from  the  scolex 
being  the  oldest.     The  last  segments  may  be  "ripe"  while  new  ones  are 
forming  below  the  scolex.     A  pair  of  lateral  nerves  extend  through  the 
body  from  segment  to  segment.     There  is  also  a  pair  of  longitudinal 
excretory  tubes  which  are  connected  by  transverse  canals  in  each  proglottis. 
Each  proglottis  contains  also  a  complete  set  of  reproductive  organs  highly 


246  CLASSIFICATION   OF   ANIMALS 

developed,  of  both  sexes.  A  "ripe"  proglottis  contains  a  large  number  of 
fertilized  eggs.  It  is  cut  off  from  the  main  chain  and  passes  from  the  host 
with  the  faeces.  The  embryonic  stages  develop  in  a  second  host  as  is 
described  in  the  case  of  a  typical  example  in  Part  III.  Page  367. 

538.  Class  IT.    Aschelminthes. — The  animals  comprised  in 
this  class  have  usually  a  cylindrical  body  and  a  simple  tubular 
digestive  tract  opening  posteriorly  by  a  vent.     A  false  body 
cavity  originating  from  the  blastula  cavity  is  often  of  consider- 
able size.     The  sexes  are  usually  distinct. 

539.  Order  i. — The  Rotatoria  are  small,  mostly  microscopic,  free-living 
animals.     They  are  found  chiefly  in  fresh  waters.     The  anterior  end  of 
the  body  is  provided  with  a  contractile  crown  of  cilia  by  which  locomotion 
is  effected.     At  the  posterior  end  there  is  usually  a  stalk-like  "foot" 
which  is  provided  with  adhesive  glands.     By  means  of  this*  foot  the 
animal  attaches  itself  temporarily.     The  name  Rotifer  has  reference  to 
the  apparent  revolution  of  the  crown  when  the  cilia  are  in  motion.     The 
currents  produced  by  the  cilia  carry  food  particles  to  the  mouth  which 
lies  in  the  centre  of  the  crown.     The  oesophagus  opens  into  a  stomach 
which  is  provided  with  a  set  of  cuticular  teeth.     A  pair  of  excretory 
tubules  opens  into  the  posterior  end  of  the  intestine. 

540.  Order  4. — The  Nematoda  are  in  part  free  living,  in  part  parasitic. 
They  are  often  called  thread  worms  or  round  worms.     The  mouth  is  at 
the  anterior    end.     There  is   then  a  sucking  oesophagus  and  a  simple 
tubular  intestine  which  opens  on  the  ventral  side  near  the  posterior  end. 
Special  sense  organs  are  practically  wanting.     There  is  a  nerve  ring  around 
the  mouth  and  from  this  a  pair  of  nerves,  one  dorsal,  one  ventral,  extend 
the  length  of  the  body.     There  is  a  pair  of  excretory  tubules,  one  on  either 
side,  which  extend  from  end  to  end  of  the  body.     The  life  histories  of 
several  parasitic  forms  are  described  in  Part  III. 

541.  Class  IV.    Nemertini. — This  group  is  composed  chiefly 
of  free-living  marine  forms.     The  body  is  much  elongated  and 
muscular.     The  epidermis  is  ciliated.     There  is  a  long  eversible 
proboscis  and  the  intestine  opens  posteriorly  by  a  vent.     Eyes 
are  often  present  and  in  large  number  and  there  is  a  pair  of 
sensory  grooves  on  the  head.     The  nervous  system  consists  of 
a  supra-cesophageal  ganglion  and  a  sub-cesophageal  ganglion. 


ANNELIDA  247 

These  are  connected  around  the  oesophagus  and  give  off  three 
longitudinal  nerves,  one  dorsal  and  two  ventral.  There  is  a 
pair  of  branched  protonephridia  which  open  on  the  side  of  the 
body.  The  sexes  are  usually  distinct. 

542.  PHYLUM    IV.     Annelida. — The    true   worms    are   free 
living,   aquatic  or,  if  terrestrial,   at  least  confined  to  moist 
situations.     The  body  is  usually  much  elongated,  bilaterally 
symmetrical   and   segmented,   and   the   segments   are   similar 
(homonomous).     The  intestine  is  usually  a  straight  tube  with 
a  vent  at  the  posterior  end  of  the  body.     There  is  a  true  body 
cavity  completely  lined  with  mesoderm.     Eyes  and  other  sense 
organs  are  often  present.     The  nervous  system  consists  of  a 
supra-cesophageal  ganglion,  a  pair  of  circum-cesophageal  con- 
nectives and  a  ventral  chain  of  ganglia  arranged  metamerically 
and  connected  by  a  pair  of  longitudinal  nerves.     In  each  seg- 
ment there  is  a  pair  of  nephridia. 

543.  Class  n.    Chsetopoda. — The  Chaetopoda   include  the 
typical  worms,  such  as  nereis  and  the  earthworms.     They  are 
distinguished  by  the  cuticular  bristles  or  setae  with  which  each 
segment  of  the  body  is  armed. 

544.  Order  i. — The  Polychceta  are  marine  annelids.     They  have  two 
bundles  of  setae  on  each  side  of  each  segment.     The  setae  are  borne  by 
short,  unjointed  appendages   (parapodia)   which  are  divided  into  two 
branches,  each  branch  having  a  bundle  of  setae.     They  usually  live  on  the 
sea  bottom  in  burrows  or  tubes  but  some  are  pelagic.     Many  are  active 
predatory  animals  and  have  well-developed  sense  organs.     Others  live  in 
leathery  or  calcareous  tubes  formed  by  secretions  of  epidermal  glands. 
These  never  leave  the  tubes  voluntarily.     Only  the  anterior  end  is  in 
most  cases  protruded  for  the  purposes  of  feeding  and  respiration.     In 
many  cases  a  circle  of  feather-like  tentacles  covered  with  cilia  produce 
currents  by  which  food  is  carried  to  the  mouth.     The  sexes  are  usually 
distinct  and  there  is  a  metamorphosis  in  development. 

545.  Order  3. — The  Oligochata  are  the  small  fresh-water  worms  and  the 
earthworms.     They  lack  parapodia  and  the  bristles  are  few  in  number, 
usually  eight  in  each  segment,  two  groups  of  two  each  on  a  side.     They 
are  hermaphrodytic  and  the  development  is  direct. 


248  CLASSIFICATION    OF    ANIMALS 

546.  Class  HI.    Hirudinea. — The  leeches  differ  considerably 
from  the  Chaetopoda.     The   external   segmentation  does  not 
correspond  to  the  metamerism.     There  are  usually  three  or 
five  external  rings  to  one  segment.     There  are  34  metameres. 
The  body  cavity  is  almost  obliterated  by  parenchyma tous  tissue. 
They  are  all  external  parasites  and  are  provided  with  suckers  for 
holding  to  the  host  and  for  locomotion.     There  are  two  suckers, 
a  small  one  at  the  anterior  end  enclosing  the  mouth  and  a  large 
one  on  the  ventral  side  at  the  posterior  end.     There  are  neither 
parapodia  nor  bristles.     Series  of  lateral  pouches  render  the 
digestive  tract  very  capacious.     The  leeches  are  hermaphrody te. 
They  are  found  only  in  the  water,  or  in  moist  places. 

547.  PHYLUN    V.      Molluscoidea. — Under    this    head    are 
grouped  several  classes  which   are   sometimes   placed   under 
the  heading  worms.     They  are  aquatic,    usually  fixed   and 
provided  with  a  cuticular  covering  in  the  form  of  a  tube  or  a 
two-valved  shell.      They  are  unsegmented.      The  mouth  is 
surrounded  by  a  circle  of  tentacles  covered  with  cilia.     The 
intestine  is  usually  U-shaped  so  that  the  vent  lies  near  the 
mouth. 

548.  Class  n.    Bryozoa. — The  Bryozoa,  or  Polyzoa,  are  all 
minute  aquatic  animals.     Most  are  marine  but  there  are  also 
a  few  fresh- water  forms.     They  are  usually  colonial  and  the 
colony  may  spread  over  considerable  areas  though  the  individ- 
uals are  barely  visible  to  the  unaided  eye.     There  is  a  gelatinous, 
horny  or  calcareous  test  into  which  the  animal  may  completely 
withdraw.     The  tests  of  a  colony  often  form  an  incrustation  over 
the  surface  of  other  objects  and  again  they  form  branching 
plant-like  structures  comparable  to  moss  plants  in  size  and 
general  appearance,  hence  the  name.     The  test  is  secreted  by 
the  epidermis  and  forms  part  of  the  animal.     A  pair  of  strong 
retractor  muscles  cause  the  rapid  withdrawal  of  the  body 
completely  within  the  test  on  the  slightest  irritation.     The 
mouth  is  surrounded  by  a  crown  of  ciliated  tentacles,  sometimes 


ANNELIDA 


249 


FIG.  139.— Amphitrite  ornata,  a  marine  Polychaete.     (From  Galloway, 

0    after  Verrill.) 


FIG.   140. — Cirratulus  grandis,  a  marine  Polychaete.     (From  Galloway 
after  Verrill.) 


250  CLASSIFICATION   OF   ANIMALS 

borne  on  a  horseshoe-shaped  disc,  the  lophophore.  The  vent 
lies  just  outside  the  tentacles.  There  are  no  special  sense  organs. 
The  nervous  system  consists  of  a  single  ganglion  and  a  few 
nerves  leading  from  it.  The  nephridia  are  very  rudimentary  or 
entirely  wanting.  The  colonies  ate  often  polymorphic;  certain 
individuals  being  specialized  receptacles  for  eggs  while  others 
form  slender,  jointed,  vibrating  whips  or  a  pair  of  jaws  which 
open  and  close  like  the  beak  of  a  bird. 

549.  Class   HI.    Brachiopoda. — The   Brachiopods   are   not 
generally  well  known,  although  they  are  not  at  all  rare  and  are 
widely  distributed.     They  live  only  in  salt  water  and  may  be 
found  not  far  from  shore.     They  have  a  shell  somewhat  like 
that  of  a  clam  but  the  plane  of  symmetry  cuts  the  hinge  at 
right  angles  and  one  of  the  valves  is  dorsal  and  the  other  ventral. 
They  are  usually  unlike.     The  shell  is  composed  either  of  chitin 
or  calcareous  matter.     The  animal  is  attached  by  a  stalk-like 
extension  of  the  posterior  end  of  the  body  or  by  the  cementing 
of  the  ventral  valve  to  the  substratum.     On  either  side  of  the 
mouth  the  body  is  prolonged  into  a  pair  of  coiled  arms  which 
bear  numerous  ciliated  tentacles.     There  is  a  true  body  cavity 
in  which  lie  the  simple  U-shaped  digestive  tract  with  a  digestive 
gland,  the  heart,  the  gonads,  one  or  two  pairs  of  nephridia  and 
a  dorsal  and  a  ventral  ganglion. 

550.  PHYLUM  VI.     Echinodermata. — This  is  a  well-defined 
group  of  animals.     They  are  all  marine,  without  exception. 
The  larvae  are  bilateral  but  by  a  metamorphosis  the  adult 
becomes   radially   symmetrical   and   the   number   of   rays  is 
usually  five.     The  name,  Echinoderm,  means  spiny  skin  and 
refers  to  the  calcareous  bars  and  plates  embedded  in  the  integu- 
ment.    These  are  not  equally  well  developed  in  the  various 
classes  but  they  serve  in  varying  degree  as  an  exoskeleton  and 
in  most  forms  some  pieces  project  beyond  the  general  surface 
in"  the  form  of  spines.     The  ambulacral  system  is  the  most 
distinctive  anatomical  character  of  the  group.     It  consists  of  a 


ECHINODERMATA 


251 


system  of  tubes  filled  with  water  which  enters  through  the 
sieve-like  madreporic  plate  near  the  aboral  pole  of  the  animal. 
From  here  a  " stone  canal"  leads  to  a  ring  canal  which  circles 
the  mouth  and  gives  off  five  radial  canals  to  the  five  rays  of  the 
body.  Each  radial  canal  has  numerous  lateral  branches  which 
end  in  tubular  processes  of  the  integument,  called  tube  feet. 


FIG.    141. — Starfish,   oral   view   showing   tube   feet.     (From   Galloway, 
after  Leuckart  and  Nitsche.) 


At  the  base  of  each  foot  there  is  a  bladder-like  lateral  enlarge- 
ment of  the  foot  canal,  the  ampulla.  By  its  contraction  the 
ampulla  forces  the  water  into  the  foot  causing  it  to  elongate. 
When  the  foot  contracts  the  water  is  forced  back  into  the 
ampulla.  The  end  of  the  foot  is  provided  with  a  sucking  disc 
by  which  it  may  be  attached  to  an  object.  By  this  mechanism, 


252 


CLASSIFICATION   OF   ANIMALS 


FIG.  142. — Section  through  the  arm  and  disc  of  a  starfish.  A,  Anus;  amp, 
ampulla;  cb,  circular  blood-vessel;  civ,  circular  water  canal;  co,  ccelom;  co.e., 
ccelomic  epithelium;  d.b.,  dermal  branchiae;  e,  eye-spot;  ect,  ectoderm;  ent,  ento- 
derm;  /,  ambulacral  foot;  g,  ambulacral  groove;  h,  hepatic  caeca;  i,  intestine; 
i.e.,  intestinal  caeca;  mes,  mesoderm;  mo,  mouth;  mp,  madreporic  body;  nr, 
nerve  ring;  os,  ossicles;  rn,  radial  nerve;  rb,  radial  blood-vessel;  rp,  genital  pore; 
rw,  radial  water  canal;  sc,  stone  canal;  sp,  spine.  (From  Galloway.) 


»d.  b. 


r.p™ 


---z 


FIG.  143. — Cross  section  of  the  arm  of  a  starfish,  ar,  Ambulacral  rafter;  ov, 
ovary  containing  ova.  Other  lettering  as  in  preceding  figure.  (From 
Galloway. ) 


ECHINODERMATA 


253 


the  numerous  feet  acting  in  unison  may  slowly  pull  the  animal 
along.  The  spines  frequently  assist  in  locomotion,  in  which 
case  they  are  long,  jointed  at  the  base  and  operated  by  special 
muscles.  In  some  cases  the  tube  feet  lack  the  sucking  disc  and 
serve  either  as  sense  organs  or 
organs  of  respiration. 

There  is  a  b'ody  cavity  in 
which  the  digestive  and  repro- 
ductive organs  are  freely  sus- 
pended. The  circulatory  system 
is  not  well  developed.  Special 
sense  organs  of  a  simple  type 
sometimes  occur.  The  nervous 
system  is  also  poorly  developed, 
consisting  chiefly  of  a  ring  of 
nerve  fibres  encircling  the  mouth 
and  sending  a  radial  nerve  into 
each  ray  of  the  body. 

551.  Class  I.  Pelmatozoa.— 
The  crinoids  or  "sea  lilies"  are 
the  only  living  representatives 
of  this  group.  The  body  is  at- 
tached by  a  stalk-like  develop- 
ment of  the  aboral  pole.  In  a 
few  forms  this  is  true  only  in 
the  young  stages  but  for  most 
families  the  stalked  condition  is 
permanent.  The  skeletal  ele- 
ments are  highly  developed.  The  stalk,  body  and  arms  are 
largely  composed  of  calcareous  joints  and  plates.  The  five  rays 
are  usually  repeatedly  branched.  The  ambulacral  system 
consists  of  ciliated  tentacles  which  serve  for  respiration  and  to 
maintain  the  currents  by  which  food  is  carried  to  the  mouth. 
In  connection  with  the  latter  function,  ciliated  grooves  on  the 


FIG.  144. — Antedon,  a  Crinoid. 
X  1/2. 


254  CLASSIFICATION   OF   ANIMALS 

oral  surface  of  the  arms  are  also  important.  The  digestive 
tract  opens  by  a  vent  on  the  oral  surface  in  an  eccentric 
position. 

552.  Class  II.     Asteroidea. — The  starfishes  lie  on  the  sea  bot- 
tom with  the  mouth  or  oral  side  down.     The  ambulacral  feet 


FIG.   145. — A  starfish,  Astropecten.     X  3/4. 

serve  for  locomotion.  The  oral-aboral  axis  is  shorter  than  a 
radius  and  the  five  radii  are  longer  than  the  inter-radii  so  that 
the  body  assumes  the  form  of  a  five-rayed  star.  The  skeleton 


ECHINODERMATA  255 

consists  of  small  rod-shaped  pieces  attached  to  each  other  by 
muscles  in  such  a  way  as  to  form  a  network.  This  skeleton  is 
very  flexible  and  by  means  of  the  muscles  the  arms  may  be 
slowly  bent  through  an  arc  of  180°  or  more.  Short  spines, 
more  or  less  movable,  project  beyond  the  general  surface. 
Each  radial  canal  ends  at  the  tips  of  the  arm  in  a  tentacle 
and  at  its  base  there  is  an  eyespot. 

553.  The  starfish  is  carnivorous,  living  largely  on  shell  fish. 
The  mouth  is  located  at  the  centre  of  the  disc  and  is  capable  of 
opening  to  an  enormous  size  so  that  large  objects  can  be  taken 
into  the  stomach.     After  digestion  the  insoluble  part  is  ejected 
at  the  mouth.     The  animal  also  attacks  larger  prey  than  it 
can  swallow.     By  attaching  some  of  the  tube  feet  to  the  solid 
substratum  and  others  to  the  two  valves  of  a  shell  fish  the 
shell  may  be  pulled  open.     Then  another  remarkable  feat  is 
performed:  The  stomach  is  everted  though  the   mouth   and 
its  rather  voluminous  folds  are  thrown  around  the  soft  parts 
of  the  prey.     Digestion  then  takes  place  outside  the  body  of 
the  animal.     Five  pairs  of  long  retractor  muscles  connect  the 
wall  of  the  stomach  with  the  interior  of  the  arms;  their  function 
is  to  draw  the  stomach  back  into  the  body.     The  stomach  is 
connected  with  a  vent  at  the  aboral  pole  by  a  small  intestine. 
But  intestine  and  vent  are  largely  functionless  and  are  often 
rudimentary.     Connected    with    the    stomach    are   five   long 
tubes  which  project  into  the  cavities  of  the  arms.     The  walls 
of  these  tubes  are  glandular  and  secrete  a  digestive  fluid. 
These  glands  are  called  hepatic  caeca.     The  sexes  are  distinct 
and  the  five  pairs  of  gonads  lie  in  the  body  cavity,  a  pair  in  each 
arm. 

554.  Class  HI.     Ophiuroidea. — The  serpent  stars,  or  brittle 
stars,  have  the  general  form  of  the  starfish  but  are  more  dis- 
tinctly divided  into  disc  and  arms.     The  arms  are  propor- 
tionately longer   and  more   slender   and   also   more   actively 
movable  and  it  is  entirely  by  the  sweeping  movement  of  the 


256  CLASSIFICATION    OF   ANIMALS 

arms  that  locomotion  is  effected.  The  ambulacral  "feet"  are 
without  sucking  discs  and  ampullae  and  do  not  assist  in  loco- 
motion. The  arms  are  almost  solid,  that  is,  the  digestive  and 
reproductive  organs  do  not  extend  into  them.  The  skeletal 
parts  are  better  developed  than  in  the  starfish.  In  the  "basket 
fish"  the  arms  are  branched. 


FIG.   146. — A  brittle  star  (Ophiuroidea).     X  3/4. 

555.  Class  IV.  Echinoidea. — In  the  sea  urchins  the  radii 
and  inter-radii  are  almost  or  quite  equal  and  hence  there  are  no 
"arms."  The  skeletal  parts  are  broad  plates  which  fit  together 
so  as  to  form  a  single  piece,  the  shell  or  test.  The  oral  surface 


ECHINODERMATA 


257 


is  down  and  the  oral  aboral  axis  is  often  nearly  equal  to  a 
horizontal  diameter  so  that  the  body  is  nearly  spherical.  The 
spines  are  often  long  and  are  movable  and  are  important  in 
locomotion.  The  tube  feet  are  much  as  in  the  starfish.  The 
true  sea  urchins  are  vegetable  feeders  and  the  mouth  is  pro- 
vided with  a  set  of  five  jaws  which  form  a  complicated  ap- 
paratus known  as  Aristotle's  lantern.  The  intestine  is  long 


FIG.  147.— The  basket  fish,  Astrophyton.     X  1/2. 

and  coiled  and  opens  by  a  vent  on  the  aboral  surface.  In 
some  groups  of  Echinoidea  a  secondary  bilateral  symmetry 
appears  and  in  these  the  vent  is  at  the  posterior  margin  of 
the  oral  surface. 

556.  Class  V.    Holothuroidea. — The  sea   cucumbers  have 
the  principal  axis  horizontal  so  the  oral-aboral  axis  is  at  right 
17 


CLASSIFICATION 


ANIMALS 


angles  to  that  of  the  other  Echinoderms.  The  principal  axis 
is  also  much  longer  than  the  radii.  The  skeletal  parts  are  re- 
duced to 'minute  hooks  and  plates,  but  the  integument  is  very 
thick  and  leathery.  The  animals  feed  on  organic  detritus 
which  they  collect  by  means  of  a  circle  of  branching  tentacles 


FIG.  148. — A  sea-urchin  (Clypeaster).  The  spines  have  been  removed.  The 
five  ambulacral  areas  are  clearly  shown.  The  test  shows  marked  bilateral 
symmetry. 

surrounding  the  mouth.  The  intestine  is  a  coiled  tube  and 
ends  at  the  aboral  pole  of  the  animal  in  a  large  cloacal  cavity. 
Lying  in  the  body  cavity  and  connected  with  the  cloaca  is  a 
very  peculiar  organ  called  the  respiratory  tree.  It  is  a  tubular 


ARTHROPOD A  259 

structure  and  is  many  times  branched.  The  main  stem  opens 
into  the  cloaca.  The  respiratory  tree  is  filled  with  water  which 
is  regularly  renewed  from  outside  through  the  cloacal  vent. 
This  is  brought  about  in  the  following  way.  Numerous  small 
muscles  connect  the  outside  of  the  cloacal  chamber  with  the 
body  wall.  When  these  contract  the  cloaca  expands  and  fills 
with  water.  The  cloacal  aperture  then  closes,  the  walls  of  the 
cloaca  contract  and  the  contents  are  forced  into  the  respiratory 
tree.  The  tubes  of  the  respiratory  tree  also  have  contractile 
walls  and  these  by  contraction  again  force  the  water  out.  In 
some  of  the  small  Holothuria,  which  do  not  possess  such  an 
impervious  integument,  the  respiratory  tree  is  absent.  The 
ambulacral  system  is  variously  developed  within  the  group. 
In  several  families  the  tube  feet  are  entirely  wanting.  Loco- 
motion is  chiefly  effected  by  worm-like  movements  of  the  body. 

557.  PHYLUM  VII.     Arthropoda. — The  Arthropoda   are   bi- 
laterally symmetrical,  segmented   animals.     They  are  distin- 
guished from  the  Annelida  by  their  jointed  appendages.     The 
number  of  segments  is  usually  not  more  than  twenty,  and  they 
are  not  alike  (heteronomous).     The  body  is  always  covered 
with  a  cuticula  of  chitin,  secreted  by  the  epidermis.     One  or 
more  pairs  of  appendages  are  modified  to  serve  as  mouth  parts 
for  the  ingestion  of  food.     The  blood  vessels  open  into  the  body 
cavity  which  is  also  connected  with  the  cavities  derived  from 
the  primitive  blastula  cavity.     The  body  is  typically  divided 
into  three  regions,  head,  thorax  and  abdomen.     The  nervous 
system  consists  of  a  brain  and  ventral  nerve  cord  as  in  the  Anne- 
lids.    In  point  of  numbers  the  phylum  includes  two-thirds  of 
the  animal  kingdom. 

558.  Class  I.     Branchiata. — As    the    name    implies    the 
Branchiata  are  the  Arthropods  which  are  provided  with  gills. 
But  this  is  true  only  of  the  larger  forms  and  even  in  some  of 
these  the  gills  have  been  lost.     With  a  very  few  exceptions, 
however,  the  Branchiata  are  aquatic.     The  term  Crust'aceae 


26o  CLASSIFICATION   OF   ANIMALS 

is  also  frequently  applied  to  the  group  because  the  chitinous 
cuticula  is  impregnated  with  salts  of  lime  which  render  it  very 
hard.  The  appendages  are  forked  (biramous)  and  at  least 
three  pairs  are  modified  as  mouth  parts  and  two  pairs,  the 
antennae,  have  a  sensory  function. 

559.  The  orders  Phyllopoda,  Ostracoda,  Branchiura  and  Copepoda  com- 
prise only  small  forms,  seldom  more  than  an  inch  in  length  and  usually 
minute.  The  Phyllopoda  are  characterized  by  their  broad,  leaf-like  swim- 
ming appendages.  The  Ostracoda  have  a  carapace  in  the  form  of  two 
valves,  like  the  shell  of  a  clam,  which  can  be  opened  and  closed.  The 
Copepoda  are  cigar  shaped  and  have  a  single  median  eye.  The  Ostracoda 
and  Copepoda  are  very  common  in  our  fresh-water  ponds.  The  Bran- 
chiura are  parasitic  in  the  gill  chambers  of  other  Crustacea  and  on  fishes. 
There  are  also  many  parasites  among  the  Copepoda. 


FIG.  149. — A  shrimp,  Palaemonetes  vulgaris.     (From  Galloway,  after  Verrill.) 


560.  Order  5. — The  Cirripedia  are  the  barnacles.  They  are  all  marine. 
The  young  are  free  swimming  but  they  come  to  rest  and  attach  them- 
selves to  some  object.  A  series  of  calcareous  plates  are  formed  by  folds 
of  the  skin.  Some  of  these  are  hinged  and  can  be  moved  by  muscles. 
The  animal  may  be  entirely  enclosed  by  the  shell.  The  appendages  are 
long  and  slender  and  are  fringed  with  hairs.  These  organs  are  thrust  out 
into  the  water  and  by  a  sweeping  motion  currents  carrying  food  particles 
are  directed  toward  the  mouth.  The  eyes  are  degenerate  in  the  adult 
but  the  reproductive  organs  are  highly  developed.  The  barnacles  are 
usually  hermaphrodytic.  Several  families  belonging  to  this  group  are 
parasitic.  The  sacculina  described  in  Part  III  is  a  notable  example. 


MALACOSTRACA  261 

561.  Order  6. — The  Malacostraca  are  a  large  group,  comprising  all  the 
larger  Crustacea.  The  head  and  thorax  together  are  composed  of  thirteen 
segments  and  the  abdomen  of  six.  There  are  always  two  pairs  of  antennae, 
a  pair  of  mandibles  and  two  pairs  of  maxillae.  In  most  cases  several  more 
pairs  of  appendages  function  as  mouth  parts.  This  order  is  very  large 
and  includes  shrimps,  prawns,  crayfish,  lobsters,  crabs,  sand  fleas  and 
"wood-lice."  Of  the  five  great  divisions  of  the  group  the  following  are 
the  most  important.  Legion  2.  Thoracostraca.  The  compound  eyes  are 
on  movable  stalks  and  the  head  and  most  or  all  of  the  segments  of  the 
thorax  are  covered  by  a  single  cuticular  shield.  The  gills  are  usually 
attached  to  the  basal  joints  of  the  thoracic  appendages  or  to  the  adjacent 
parts  of  the  body-wall  and  are  covered  by  the  cephalo-thoracic  shield. 


FIG.  150. — Caprella  geometrica,  an  Amphipod.     (From  Galloway,  after  Verrill.) 

The  sub-order  Decapoda  comprises  the  most  important  families.  In  this 
group  the  five  pairs  of  appendages  from  the  gth  to  i3th  segments  inclusive 
are  ambulatory  appendages.  Those  of  the  3rd  to  8th  are  mouth  parts. 
The  following  analysis  of  the  classes  of  the  malacostraca  will  show  the 
relation  of  the  groups  mentioned. 
Order,  Malacostraca. 

Legion  i.  Leptostraca. 
Legion  2.  Thoracostraca. 

Sub-order  i.  Schizopoda. 
Sub-order  2.  Decapoda. 

Section  i.  Macrura  natantia,  shrimps,  pawns. 
Section  2.  Macrura  reptantia,  crayfish,  lobster. 
Section  3.  Anomura,  hermit  crabs. 
Section  4.  Brachyura,  crabs. 
Sub-order  3.  Cumacea. 
Legion  3.  Stomatopoda. 
Legion  4.  Anomostraca. 
Legion  5.  Arthrostraca. 

Sub-order  i.  Anisopoda. 

Sub-order  2.  Isopoda,  "pill  bug"  =  " wood-louse." 

Sub-order  3.  Amphipoda,  sand  fleas. 


262  CLASSIFICATION  OF   ANIMALS 

562.  The  Macrura  natantia  (swimming  large  tails)  are  generally  com- 
pressed laterally,  the  ambulatory  appendages  are  slender  and  the  animal 
depends  chiefly  on  the  backward  stroke  of  the  strong  abdomen  for  loco- 
motion.    The  Macrura  reptantia  (crawling  large  tails)  are  not  compressed 
and  the  thoracic  appendages  are  strong  as  is  also  the  abdomen.     The 
section  Anomura  includes  the  hermit  crabs  which  live  habitually  with 
the  abdomen  thrust  into  an  empty   snail  shell.     For  this  reason  the 
abdomen  is  twisted,  the  posterior  thoracic  appendages  reduced  and  the 
caudal  fin  transformed  into  an  unsymmetrical  hold-fast    organ.     The 
Brachyura  are  the  true  crabs  in  which  the  abdomen  is  much  reduced  and 
turned  forward  under  the  cephalo-thorax. 

563.  The  Arthrostraca  have  sessile  eyes  and  the  thoracic  shield  is  almost 
or  wholly  wanting.     In  the  Isopoda  the  body  is  flattened  dorso-ventrally 
and  there  are  no  gills  on  the  thoracic  appendages.     The  Amphipoda  are 
compressed  laterally  and  gills  are  present  on  the  thoracic  appendages. 
The  body  is  curved  with  the  abdomen  turned  down  and  forward.     The  last 
appendages  of  the  abdomen  are  turned  backward  and  are  used  in  connec- 
tion with  the  backward  stroke  of  the  abdomen  to  spring  the  body  forward 
analogous  to  the  movement  of  a  flea,  hence  the  name.     Most  of  the 
Malacostraca  are  marine.     The  crayfishes  are  fresh-water  representatives 
of  the  order.     A  number  of  crabs  live  on  land  though  usually  in  the 
vicinity  of  water.     The  wood-louse  is  also  a  familiar   terrestrial  form, 
though  it  is  also  at  home  only  in  damp  places.     The  terrestrial  crabs  have 
rudimentary  gills  and  the  gill  chamber  takes  on  the  function  of  a  lung. 

564.  Class  n.    Palaeostraca. — These  arthropods  have  only 
one  pair  of  appendages  anterior  to  the  mouth  and  five  pairs 
surrounding  the  mouth.     Those  around  the  mouth  serve  both 
for  the  ingestion  of  food  and  for  locomotion. 

565.  Order  2. — The  Xiphosura  have  a  broad  horseshoe-shaped  cephalo- 
thorax,  an  unjointed  abdomen  and  a  long  spine-like  terminal  appendage, 
to  which  the  name  "sword  tail"  refers.     There  are  a  pair  of  ocelli  and  a 
pair  of  compound  eyes  on  the  dorsal  surface  of  the  cephalothorax.     The 
first  pair  of  appendages  are  chelate  but  are  not  used  in  locomotion.     The 
following  five  pairs  are  locomotor  but  their  basal  joints  are  spiny  and 
serve  for  the  trituration  of  the  food.     The  abdominal  appendages  are 
broad  leaf -like  structures  which  protect  the  "book  gills"   which   are 
attached  to  them.     There  is  only  one  genus  of  Xyphosura,  the  horseshoe 
crab,  Limulus. 


PALAEOSTRACA 


263 


Knm'. 


FIG.  151.— Limulus,  the  horse-shoe  crab.     Dorsal  view.     (From  Patten.) 


264 


CLASSIFICATION   OF   ANIMALS 


566.  Class  HI.    Arachnoidea. — The  Arachnoidea   are   air- 
breathing  Arthropods,   with   a   cephalothorax,   two   pairs    of 
mouth  appendages  and  four  pairs  of  locomotor  appendages. 
The  abdomen  has  no  appendages.     There  are  from  2  to  1 2  eyes. 

567.  Order  i. — The  Scorpionidea  are  large  Arachnoidea  with  a  long 
segmented  abdomen  consisting  of  a  preabdomen  of  seven  segments  and 


FIG.  152. — A  Scorpion,  Buthus  afer.     X  1/2. 


a  postabdomen  of  six  segments  and  ending  in  a  spine  and  poison  gland. 
The  first  and  second  pairs  of  appendages,  the  chelicerae  and  maxillary 
palps  are  chelate.  The  respiratory  organs  consist  of  four  pairs  of  book 
lungs  which  open  on  the  ventral  side  of  the  preabdomen. 

568.  Order  2. — The  Pedipalpi  have  clawed  chelicerae,  clawed  or  chelate 


ARACHNOIDEA  265 

maxillary  palps;  and  the  third  pair  of  appendages  are  whip  like.     The 
abdomen  is  11-12  jointed.     There  are  two  pairs  of  book  lungs. 

569.  Order  3. — The  Araneida,  or  true  spiders,  have  the  abdomen  con- 
nected with  the  cephalothorax  by  a  narrow  waist.  The  abdomen  is 
usually  unsegmented.  The  chelicerae  are  chelate  and  the  maxillary  palps 
are  similar  to  the  ambulatory  appendages.  There  are  four  book  lungs 
or,  two  book  lungs  and  two  tracheae  or,  four  tracheae.  On  the  ventral 
surface  of  the  abdomen  is  a  group  of  four  or  six  glands  from  which  the 


FIG.  153. — The  great  bird-killing  spider,  Mygale,  of  South  America.      X  5/8. 

web  is  spun.  The  web  is  a  fluid  secretion  which  sets  on  exposure  to  the 
air.  It  is  used  to  build  and  line  the  nest,  for  building  traps  by  which 
the  prey  is  caught,  for  winding  about  the  egg  masses,  for  locomotion  and 
a  variety  of  other  purposes.  The  chelicerae  are  provided  with  a  poison 
gland. 

570.  Order  6. — The  Opilionidea  are  the  "harvestmen"  or  " daddy-long- 
legs." The  abdomen  is  broadly  connected  with  the  cephalo-thorax  so 
that  the  whole  forms  apparently  one  continuous  body.  The  chelicerae 
are  chelate  and  the  maxillary  palpi  are  like  the  legs.  The  legs  are  often 
very  long.  The  respiratory  organs  are  tracheae. 


266 


CLASSIFICATION   OF   ANIMALS 


FIG.  1 54;! . — Home  of  the  trap-door  spider.     Door  closed. 


FIG.  I54-B. — Home  of  the  trap-door  spider.     The  door  propped  open 
with  a  straw.     X  i. 


ARACHNOIDEA 


267 


571.  Order  8. — The  Acarina  are  the  mites  and  ticks.  Many  of  the 
members  of  this  group  are  parasites.  The  body  cannot  be  divided  into 
cephalo-thorax  and  abdomen  as  all  evidence  of  segmentation  is  lacking. 
The  mouth  parts  are  often  adapted  for  piercing  and  sucking.  The  legs 


FIG.  155. — The  home  of  the  trap-door  spider  laid  open.     The  door  is  held  open 
with  a  pin.     X  3/4- 

are  often  merely  hold-fast  organs.     The  parasitic  forms  are  often  much 
degenerate. 

572.  Order  9. — The  Linguatulida  are  parasitic  forms  especially  notable 


268 


CLASSIFICATION   OF   ANIMALS 


for  the  degenerated  condition  of  many  organs.     The  body  is  worm-like 
and  the  appendages  are  reduced  to  two  pairs  of  hooks. 

573.  Class  6.    Protracheata.— This  group  is  of  interest  as 
forming  a  connecting  link  between  the  annelids  and  insects. 


FIG.  156.— The  trap-door  spider.     X  i. 

There  are  only  a  few  species,  which  are  not  common  but  are 
found  in  widely  separated  parts  of  the  globe.  The  animals  are 
small,  worm-like,  and  are  found  in  damp  places  under  stones, 
decaying  wood  or  other  similar  situations.  The  body  is  worm- 


FIG.  157, — Peripatus  capensis,  an  example  of  the  class  Protracheata. 
(From  Galloway,  after  Moseley.) 

like  and  provided  with  short  feet  somewhat  like  the  false  feet 
of  a  caterpillar  but  provided  with  claws.  There  is  one  pair  of 
tentacles  and  a  pair  of  eyes.  There  are  two  pairs  of  appendages 
in  the  region  of  the  mouth.  The  respiratory  organs  are  tracheae 
with  stigmata  scattered  irregularly  over  the  surface  of  the  body. 


MYRIAPODA 


269 


574.  Class  7.  Myriapoda. — The  "thousand  legs"  have  a 
worm-like  body  divided  into  similar  segments,  a  distinct  head 
with  one  pair  of  antennae,  usually  one  pair  of  maxillae  and  with 
one  or  two  pairs  of  appendages  on  each  body  segment .  Respira- 
tion is  by  tracheae  and  the  stigmata  are  arranged  segmentally. 


575.  Order  3. — The  Diplopoda   are    the 
common   "thousand   legs."     The   body  is 
cylindrical     or     half-cylindrical     and     is 
covered  with  a  chitinous  cuticula  hardened 
by  deposits  of  carbonate  of  lime.     All  the 
segments  except  a  few  of  the  most  anterior 
and   the   last   one   bear   two  pairs  of  ap- 
pendages.     The     animals     are    vegetable 
feeders  and  harmless. 

576.  Order  4. — The  Chilopoda  bear  some 
resemblance   to   the  preceding  group  but 


FIG.  158. — Spirobolus,  a  Diplopod. 
(From  Folsom.) 


FIG.  159. — Campodea,  an  example 
of  the  class  Apterygogenea.  (From 
Folsom.) 


the  body  is  usually  flattened  and  no  segment  bears  more  than  one  pair  of 
appendages.  There  is  a  pair  of  mandibles,  two  pairs  of  maxillae  and  one 
pair  maxillipeds.  The  maxillipeds  belong  to  the  first  body  segment.  They 
are  stout  claws  and  contain  a  poison  gland.  The  centipede  of  the  south 
is  an  example  of  this  order  but  a  more  familiar  one  is  the  long-legged 
Cermatia  often  seen  in  our  dwellings  where  it  preys  upon  other  insects. 


270 


CLASSIFICATION   OF   ANIMALS 


577.  Class  9.  Apterygogenea. — This  class  is  often  grouped 
with  the  insects  as  the  lowest  order.  They  have  many  char- 
acters in  common  with  insects  and  are  doubtless  closely  related. 


FIG.  1 60. — Mouth  parts  of  a  cockroach,  Ischnoptera  pennsylvanica.  A, 
labrum;  B,  mandible;  C,  hypopharynx;  D,  maxilla;  E,  labium;  c,  cardo;  g  (of 
maxilla)  galea;  g  (of  labium)  glossa;  /,  lacinia;  Ip.  labial  palpus;  m,  mentum; 
mp,  maxillary  palpus;  p,  paraglossa;  pf,  palpifer;  pg,  palpiger;  s,  stipes;  sm, 
submentum.  B,  D  and  E  are  in  ventral  aspect.  (From  Folsom.) 


FIG.  161. — Alimentary  tract  of  a  grasshopper,  Melanoplus.  c,  Colon;  cr,  crop; 
gc,  gastric  caeca;  i,  ileum;  m,  midintestine,  or  stomach;  ml,  malpighian  tubules; 
o,  oesophagus;  />,  pharynx;  r,  rectum;  s,  salivary  gland.  (From  Folsom.) 

There  is  a  distinct  head,  a  thorax  of  three  segments  and  ai 
abdomen  of  6-1 1  segments.     The  head  bears  a  pair  of  antennae 


APTERYGOGENEA 


271 


and  sometimes  compound  eyes.  An  ocellus  may  also  be  pres- 
ent. There  is  a  mandible  and  two  pairs  of  maxillae.  Each 
segment  of  the  thorax  bears  a  pair  of  appendages.  The  only 
evidence  of  abdominal  appen- 
dages to  be  found  are  a  pair  of 
stiff  bristles  attached  to  the 
ventral  surface  of  the  fifth 
segment  and  projecting  forward 
beneath  the  body,  and  a  pair  of 
hooks  beneath  the  third  seg- 
ment. By  means  of  this  ap- 
paratus the  animal  makes 
springing  movements.  Hence 
they  are  called  spring  tails.  The 
respiratory  organs  are  tracheae 
except  in  some  groups  where 
respiratory  organs  are  wanting. 

578.  Class  10.    Insecta. — In 
the  Insects  the  body  is  divided 
into  head,  thorax  of  three  seg- 
ments   (prothorax,  mesothorax 
and    metathorax)    and  an  ab- 
domen of  9-10  segments.     The 
appendages  of  the  head  are  a 
pair  of  antennae,  a  pair  of  man- 
dibles, a  pair  of   maxillae   and 
a   labium   which    represents    a 
second  pair  of  maxillae.      Each 
segment  of  the  thorax  bears  a 
pair   of   legs    and   in   addition 
the  meso-  and  metathorax  also 

each  have  a  pair  of  wings.     The  abdominal  segments  bear  no 
appendages. 

579.  The  mouth  opens  into  a  narrow  oesophagus  with  which 


FBG.  162 — Tracheal  system  of  an 
insect,  a,  Antenna;  b,  brain;  I,  leg; 
n,  nerve-cord;  p,  palpus;  s,  spiracle; 
st,  spiracular,  or  stigmatal  branch; 
/,  main  tracheal  trunk;  v,  ventral 
branch;  vs,  visceral  branch.  (From 
Folsom,  after  Kolbe.) 


272  CLASSIFICATION   OF   ANIMALS 

several  salivary  glands  are  connected.  The  oesophagus  is  often 
enlarged  to  form  a  crop.  In  several  orders  there  is  also  a 
gizzard  between  the  crop  and  the  stomach.  The  stomach  is 
the  true  digestive  portion  of  the  alimentary  canal.  It  is  larger 
in  diameter  than  oesophagus  or  intestine  and  usually  has  sack- 
like  or  tubular  glands  opening  into  its  anterior  end.  Following 
the  stomach  is  first  a  narrow  small  intestine  and  then  a  wider 
large  intestine.  At  the  junction  of  stomach  and  small  intestine 
there  are  a  number  of  long  and  very  slender  tubes,  known  as 
Malpighian  tubules.  These  are  the  excretory  organs  of  the 
insect.  The  respiratory  system  consists  of  a  greatly  branched 
system  of  trachea.  These  open  on  the  surface  at  the  side  of  the 
abdomen  and  thorax.  The  tracheal  capillaries  extend  to  all 
parts  of  the  body.  The  heart  is  a  long  contractile  vessel  lying 
on  the  dorsal  side  of  the  abdomen.  The  blood  enters  it  through 
eight  pairs  of  ostia  segmentally  arranged.  A  vessel  leads 
forward  from  the  heart  into  the  head.  The  blood  circulates 
through  the  body  cavity. 

580.  There  are  usually  a  pair  of  highly  developed  compound 
eyes  and  sometimes  two  or  three  ocelli.     Other  special   sense 
organs  are  the  tactile  hairs  on  the  antennae,  the  olfactory  cones 
and  pits  of  the  palpi  and  taste  cells  of  the  mouth  cavity.     The 
"brain"  is  large  and  complex  in  structure.     The  ventral  gang- 
lionic  chain  may  in  reality  be  a  chain  of  as  many  as  twelve 
ganglia,  but  various  stages  of  concentration  occur  even  to  the 
fusion  of  all  into  one  mass. 

581.  The  gonads  open  by  a  pair  of  ducts  at  the  posterior 
end  of  the  abdomen.     The  sexes  are  separate  and  usually  di- 
morphic.    In  some  orders  polymorphism  is  not  uncommon. 
The  eggs,  in  many  cases,  develop  without  fertilization  (par- 
thenogenesis) .     In  most  orders  there  is  a  marked  metamorphism. 

582.  Order  i. — The  Orthoptera  are  insects  with  biting  mouth  parts,  two 
pairs  of  wings  which  are  unlike,  and  development  by  an  incomplete  meta- 
morphosis.     The    order    includes,   earwigs,   cockroaches,   the  praying- 


INSECTA  273 

mantis,  "devil's  horse"  or  "darning  needle,"  grasshoppers,  katydids  and 
crickets, 

583.  Order  3. — The  Corrodentia  have  biting  or  rudimentary  mouth  parts, 
wings  alike,  and  development  without  or  with  little  metamorphosis.     The 
group  includes  the  highly  interesting  "white  ants"  or  termites  and  the 
less  interesting  body  lice,  ectoparasitic  on  mammals.     The  latter  are 
degenerate,  lacking  wings  and  having  rudimentary  mouth  parts  and  eyes 
greatly  reduced.     The  termites  are  colonial  and  polymorphic.     The  sexu- 
ally perfect  males  and  females  are  winged  but  the  wings  are  later  cast  off. 
A  third  form  called  a  worker  and  sometimes  another  form  called  a  soldier 
may  also  be  found.     These  are  individuals  in  which  the  reproductive 
system  remains  undeveloped. 

584.  Orders  6  and  7. — The  Odonata,  "damsel  flies,"  dragon  flies,  and 
Ephemeroidea,  "day  flies,"  are  found  only  in  the  vicinity  of  fresh-water 
ponds  or  streams  in  which  the  larval  development  takes  place. 

585.  Order  8. — Some  of  the  Neuroptera,  "lace  wings,"  also  develop  in 
the  water  as  for  example  Corydalis   whose  larva  is  the  "hellgrammite." 
The  ant  lion  ("doodle  bug")  is  the  larva  of  another  "lace  wing."     The 
Odonata  have  an  incomplete,  the  Ephemeroidea  and  Neuroptera  a  complete 
metamorphosis.     All  the  remaining  Orders  except  the  last,  Rhynchota, 
also  undergo  complete  metamorphosis. 

586.  Order  n. — The  large  order  Lepidoptera,  butterflies  and  moths,  is 
perhaps  the  most  readily  distinguished  of  all.     Here  the  maxillae  are 
modified  for  sucking  and  form  a  proboscis.     The  two  pairs  of  wings  are 
similar  and  covered  with  scales.     The  prothorax  is  united  firmly  with  the 
mesothorax.     The  larva  is  a  caterpillar  with  distinct  head  and  jaws  devel- 
oped for  biting.     The  head  also  bears  two  antennae  and  two  or  three  pairs 
of  simple  eyes.     The  first  three  segments  behind  the  head  have  jointed 
appendages  and  there  are  besides  on  the  segments  of  the  abdomen  from 
two  to  five  pairs  of  false  feet.     After  a  time  of  voracious  feeding  and  rapid 
growth  the  larva  attaches  itself  in  some  sheltered  place  or  spins  a  cocoon 
of  silk  fibres.     It  then  undergoes  a  complete  change  of  form,  meta- 
morphosis, becoming  a  quiescent  pupa  in  which  condition  it  continues 
for  a  short  time  if  it  is  in  the  summer  or  through  the  winter  if  pupation 
takes  place  in  the  fall.     Finally  another  transformation  takes  place,  the 
integument  of  the  pupa  bursts  and  the  imago  emerges  in  all  respects  a 
mature  insect.     The  life  period  of  the  imago  is  usually  brief;  the  female 
is  fertilized,  deposits  a  single  brood  of  eggs  and  dies. 

587.  Order  12, — The  Diptera  or  two  wings  include  the  flies,  gnats,  and 
mosquitos.     They  have  mouth  parts  developed  for  sucking  or  piercing. 

18 


274 


CLASSIFICATION   OF   ANIMALS 


The  anterior  wings  are  membraneous,  the  posterior  pair  reduced  to  "bal- 
ancers" or  halteres.  The  body  is  usually  compact  with  the  ventral  chain 
of  ganglia  united  into  a  single  mass.  The 
abdomen  consists  of  5-9  segments.  The  larva 
is  a  footless  and  often  headless  grub  (maggot) 
and  in  the  process  of  metamorphosis  is  trans- 
formed into  a  pupa  and  finally  the  imago. 
The  larvae  of  the  mosquitos  are  aquatic,  those 
of  most  of  the  true  flies  live  in  decaying  organic 
matter  but  many  are  parasitic.  In  a  number 
of  cases  the  adult  is  also  parasitic. 

588.  Order   13. — The   Siphonaptera  or  fleas. 
In  this  group  the  wings  are  wanting  through 
degeneration.     Compound  eyes  are  also  lack- 
ing.    The  body  is  laterally  compressed.     The 
mouth    parts    are    for    piercing    and  sucking. 
The  third  pair  of  legs  are  used  for  springing. 
The  larvae  are  usually  free  living,  the  adult  an 
external  parasite. 

589.  Order  14. — The  Coleoptera,  or  beetles, 
are  a  very  large  order.     The  mouth  parts  are 
constructed  for  biting.     The  anterior  pair  of 
wings  are  horny,  the  second  pair  membraneous. 
The   first   pair    are   called   elytra.      They   fit 
together   to   form* a  shield  over  the  abdomen 
and  at  rest  the  second  pair  of  wings  are  folded 
under  them.     The  larvae  have  a  distinct  head 
with    simple    eyes   and    a  soft  body — ("grub 
worms").      The  feet  may  be   wanting.     The 
grub  lives  in  protected  situations,  underground, 
under  the  bark  of  trees  or  boring  into  wood  or 
in  other  similar  places.     Metamorphosis  in- 
cludes a  pupa  stage. 

590.  Order    16. — The    Hymenoptera    include 
the  ants,  bees  and  wasps.      The  mouth  parts 
are  adapted  for  biting.     The  wings  are  two 

There  are 


FIG.  163. — The  Lantern- 
fly  of  Brazil.  Fulgora 
lanternaria.  This  odd  ex- 
ample of  the  Rhynchota 
is  said  by  the  natives  to 
carry  a  light  in  the  pecu- 
liar appendage  borne  on 
the  head.  This  statement  pairs,  of  a  membraneous  texture. 


is  seriously  questioned,  two  compound  eyes  and  three  ocelli.  The 
of™Cthne»Tr  females  are  provided  with  a  sting  of  a  complex 
known.  X  5/4.  structure  and  located  at  the  posterior  end  of 


INSECTA  275 

the  abdomen.  This  may  be  used  for  depositing  eggs  or  for  defense. 
The  brain  is  highly  developed.  Some  larvae  feed  on  leaves,  others  are 
parasitic  in  the  tissues  of  other  insects  or  of  plants  while  others  are 
fed  by  the  adults  with  either  animal  or  vegetable  food.  The  larvae 
usually  spin  a  cocoon  in  which  the  pupa  stage  is  passed.  Some  of  the 
most  important  forms  are:  The  gall  wasps  which  deposit  the  eggs  in 
the  tissues  of  plants  whereby  a  gall  develops  and  forms  a  shelter  and 
source  of  food  for  the  larvae;  the  ichneumon  flies  which  sting  the 
larvae  of  other  insects  and  deposit  their  eggs  there,  the  larvae  then 
developing  as  internal  parasites;  the  ants  with  their  complex  social  organi-r 
zation,  polymorphism,  consisting  of  three  or  four  types  of  individuals, 
and  division  of  labor,  keeping  of  slaves,  cultivation  of  plants  and  fostering 
of  aphids  for  economic  purposes;  the  common  solitary  wasps  and  social 
wasps  with  the  more  or  less  artfully  constructed  nests  of  mud  or  " paper"; 
the  social  and  polymorphic  honey  bee  and  the  bumble  bees  with  their 
combs  and  honey.  This  interesting  order  merits  a  special  treatise. 

591.  Order  17. — The  Rhynchota  or  bugs.     These  insects  are  provided 
with   a  protruding    snout   and  piercing   mouth  parts.     Metamorphosis 
occurs  in  some  cases,  in  variable  degree.     The   wings  are  sometimes 
wanting  but  there  are  usually  two  pairs.     The  anterior  pair  may  be 
partly  horny.     They  are  all  ectoparasitic  on  other  animals  or  plants. 
Included  in  this  group  are  the  bed  bugs,  the  plant  bugs,  such  as  the 
squash  bug,  chinch  bug  and  cicada,  the  water  bugs,    water-boatmen, 
water-striders  and  electric-light   bug,  and   the  "plant  lice"  and  scale 
insects. 

592.  PHYLUM  VIII.    Mollusca. — The  Molluscs  are  the  highest 
group   of  unsegmented   animals.     The  group  is  pretty  well 
denned  but  there  is  a  great  difference  in  the  scale  of  organization 
between   the  lowest  and  highest  orders.     The  most  marked 
anatomical  character  of  the  phylum  is  the  mantle,  which  is  a 
single  or  paired  fold  of  the  integument  of  the  dorsal  side  of  the 
body.     This  mantle  is  usually  of  sufficient  extent  to  entirely 
enclose  the  body  of  the  animal,  and  on  its  external  surface  it 
secretes  a  hard  shell  composed  of  horny  and  calcareous  matter 
deposited  in   layers.     There   are   no   paired   appendages.     A 
ventral  muscular  portion  of  the  body  is  called  the  foot  and 
sometimes  serves  for  locomotion.     The  nervous  system  con- 


276  CLASSIFICATION   OF   ANIMALS 

sists  of  three  ganglia  called  the  cerebral,  pedal  and  visceral 
ganglia.  .These  are  connected  by  paired  nerves.  The  ccelomic 
cavity  is  almost  obliterated  by  a  mesenchymatous  parenchyma. 
A  pair  of  nephridia  connect  the  remnant  of  the  body-cavity 
with  the  exterior.  There  is  also  a  pair  of  gonads.  The  body 
is  fundamentally  bilaterally  symmetrical  but  in  one  large  group 
a  twisting  of  the  visceral  mass  results  in  more  or  less  asymmetry. 
The  mollusca  are  primarily  aquatic  animals  but  a  large  number 
have  become  adapted  to  a  terrestrial  life. 

593.  Class  I.    Amphineura. — This  is  a  small  class  and  only 
one  example  need  be  described.     Chiton  is  bilaterally  sym- 
metrical and  flattened  dorso-ventrally.     There  is  a  partially 
differentiated  head  but   the   mantle  fold   includes    the   head 
as  well  as  the  body.     On  its  dorsal  surface  the  mantle  forms  a 
single  series  of  eight  plates.     The  foot  is  very  broad  and  mus- 
cular and  is  used  for  locomotion  and  as  a  sucking  disc  for  a 
hold-fast.     In  the  groove  between  the  mantle  and  the  side  of 
the  body  is  a  series  of  ctenidia,  or  comb-like  gills.     The  digestive 
tract  consists  of  a  mouth  cavity  with  radula  and  a  pair  of 
" salivary"   glands,  an   oesophagus,  stomach   with   a  pair   of 
digestive  glands  and  a  coiled  intestine.     The  vent  is  at  the 
posterior  end,  opening  into  the  mantle  cavity.     The  nervous 
system  consists  of  an  cesophageal  ring  or  nerve  collar  and  two 
pairs  of  longitudinal  nerves,  one  ventral  and  one  lateral.     Be- 
sides, there  are  several  smaller  ganglia  and  numerous  connectives 
between  the  longitudinal  nerves.     There  is  a  pair  of  nephridia 
and  a  double  gonad  with  paired  ducts.     All  the  Amphineura 
are  marine. 

594.  Class    IT.     Conchifera. — This    group    is   distinguished 
from  the  preceding  by  the  fact  that  the  mantle  fold  does  not 
include  the  head  and  by  the  way  in  which  the  shell  is  formed. 
The  latter  consists  of  numerous  spine-like  pieces  in  the  Amphi- 
neura while  in  the  Conchifera  it  is  formed  in  layers  as  a  single 
structure. 


MOLLUSC  A  277 

595.  Order  i. — The  Gastropoda  have  a  distinct  head,  a  twisted  visceral 
sack,  a  single  shell  and  a  creeping  foot  with,  sometimes,  lateral  swimming 
lobes.  The  head  usually  bears  two  or  four  tentacles  and  a  pair  of  eyes. 
The  visceral  organs  are  chiefly  contained  in  a  dorsal  conical  sack-like 
development  of  the  body-wall  which  is  more  or  less  coiled  in  form  of  a 
spiral.  The  lower  edge  of  the  mantle  forms  a  collar-like  continuation  of 
the  ventral  edge  of  the  visceral  sack.  The  entire  surface  of  the  visceral 
sack  down  to  the  edge  of  the  collar  is  covered  with  a  calcareous  shell  into 
which  the  head  and  foot  may  also  be  withdrawn.  Between  the  surface 
of  the  mantle  and  the  body  there  is  a  space  of  considerable  size.  This  is 
called  the  mantle  cavity.  In  it  lie  two  feather-like  gills,  ctenidia,  which 
are  developed  from  the^body-wall.  Because  of  the  twisting  of  the  vis- 
ceral mass  the  gills  become  shifted  in  position  and  one  is  frequently 
reduced. 


FIG.  164. — The  garden  snail,  Helix.  A,  The  shell  in  section,  a,  Apex;  an, 
anus;  ap.  aperture;  c,  columella;  e,  eyestalk;  /,  foot;  /,  lip;  m,  edge  of  mantle 
(collar);  ra,  respiratory  aperture;  s,  suture;  /,  tentacles.  (From  Galloway.) 

596.  In  the  mouth  cavity  there  is  a  horny  jaw  and  a  tongue  covered 
with  a  rough,  file-like  cuticular  ribbon.  This  as  called  a  radula.  The 
digestive  tract  is  rather  long  and  coiled.  There  are  a  pair  of  "salivary" 
glands  with  ducts  opening  into  the  buccal  cavity.  The  oesophagus  is 
enlarged  into  a  crop.  Another  enlargement  of  the  canal  forms  a  stomach 
into  which  the  ducts  of  a  large  digestive  gland  ("liver")  open.  The 
coiled  small  intestine  opens  into  a  shorter  and  wider  large  intestine. 


278  CLASSIFICATION    OF   ANIMALS 

The  vent  is  usually  on  the  right  side  anterior  to  the  visceral  mass.  The 
heart  consists  of  a  ventricle  and  one  or  two  auricles.  It  lies  in  a  small 
body  cavity  called  a  pericardial  chamber.  The  kidney  communicates 
with  the  pericardial  chamber  by  a  nephridial  funnel  and  opens  into  the 
mantle  chamber  through  a  duct — the  ureter. 

597.  The  nervous  system  consists  of  a  pair  of  cerebral  ganglia,  pleural 
ganglia  and  pedal  ganglia  which  are  all  closely  connected  into  a  nerve 
collar.     There  are  also  parietal  and  buccal  ganglia.     From  these  ganglia 
nerves  are  supplied  to  the  sense  organs  and  muscles  of  the  head,  to  the 
mouth,  to  the  foot,  to  the  gills,  olfactory  organs  (osphradia)  and  a  part 
of  the  mouth,  and  to  the  buccal  mass  and  intestine  respectively.     The 
sense  organs  usually  present  are  tentacles,  eyes,  a  statocyst  which  is 
usually  close  by  the  pedal  ganglion  though  it  is  innervated  from  the  brain, 
and  chemical  sense  organs,  called  osphradia,  located  on  or  near  the  gills. 

598.  Some  of  the  Gastropods  are  hermaphroditic,  in  others,  the  sexes 
are  separate.     The  reproductive  system  is  frequently  very  complicated 
for  besides  the  gonads  and  their  ducts  which  may  be  variously  modified, 
there  may  be  two,  three  or  more  kinds  of  glands  and  other  accessory 
reproductive  organs.     Development  is  either  direct  or  by  metamorphosis. 
The  embryo  is  at  first  symmetrical.     A  larva  known  as  a  veliger  occurs 
in  many  forms. 

599.  The  Gastropods  are  typically  aquatic  but  there  are  many  forms 
in  which  the  mantle  chamber  serves  as  a  lung,  no  gills  being  developed. 
This  is  the  case  with  many  fresh-water  forms  and  a  large  number  of 
forms   which  are  purely   terrestrial.     Many   Gastropods   are   vegetable 
feeders.     Others  are  carnivorous,  some  have  the  power  of  boring  through 
the  shells  of  other  molluscs  by  means  of  an  acid  secretion,  and  thus  killing 
their  prey. 

600.  The  numerous  families  of  Gastropods  are  classified  as  follows: 

Legion  I.  Strep toneura 

Sub-order  i.  Aspidobranchia 

Sub-order  2.  Ctenobranchia 

Sub-order  3.  Heteropoda 
Legion  II.  Euthyneura 

Sub-order  i.  Opisthobranchia 

Sub-order  2.  Pulmonata. 

601.  The  Streptoneura  have  the  visceral  nerves  crossed  like  a  figure  8 
because  of  the  twisting  of  the  visceral  mass.     For  the  same  reason  the 
gills  lie  in  front  of  the  heart.     The  sexes  are  generally  distinct.     The 


MOLLUSC  A  279 

Aspidobranchia  have  feather-shaped  (double)  gills  which  are  free  at  the 
tips.  The  Ctenobranchia  have  a  single  comb-shaped  (single)  gill.  The 
Heteropoda  are  pelagic.  The  foot  forms  a  flat  fin.  The  visceral  mass  is 
small  and  the  shell  poorly  developed  or  wanting. 

602.  The  Euthyneura  have  the  visceral  nerves  parallel.     They  are 
hermaphrodytic.     The  Opisthobranchia  are  marine  forms  with  the  gills 
usually  behind  the  heart.     The  Pulmonata  are  chiefly  terrestrial  and 
fresh  water  snails  without  gills.     The  mantle  cavity  serves  as  a  lung.     In 
the  slugs  the  shell  is  greatly  reduced  or  wanting. 

603.  Order  2. — The  Solenoconcha  have  a  horn-shaped  shell  and  body 
and  a  cylindrical  foot.     The  group  is  small  and  the  animals  are  also  small. 
They  are  marine  and  live  in  the  mud  of  the  bottom. 


FIG.   165. — A  slug,  Limax.     (From  Galloway,  after  Binney's  Gould.) 

604.  Order  3. — The  Lamellibranchiata  are  compressed  laterally.     The 
head  is  rudimentary.     The  mantle  is  large  and  double,  right  and  left. 
The  shell  is  also  double  and  the  two  valves  are  connected  by  a  dorsal 
ligament.     The  foot  is  usually  wedge  shaped.     There  are  two  pairs  of 
plate-like  gills.     The  animal  is  usually  bilaterally  symmetrical  but  there 
may  be  considerable  deviation  from  this  rule. 

605.  The  shell  is  secreted  by  the   mantle  and  is  composed  of  three 
layers.     On  the  surface  is  a  thin  layer  of  a  horny  cuticula  (periostracum) 
which  is  formed  by  the  extreme  edge  of  the  mantle.     The  hinge  ligament 
is  of  the  same  substance  but  forms  a  very  thick  layer.     The  hinge  is  elastic 
and  causes  the  shell  to  gape   when  the  adductor  muscles  are  relaxed. 
Beneath  the  cuticula  there  is  a  thick  layer  of  calcium  carbonate  deposited 
in  a  matrix  of  organic  matter  (conchiolin).     The  limey  portion  of  the  shell 
consists  of  two  layers,  an  outer  "prismatic"  layer  and  an  inner  layer  of 
"mother  of  pearl."     The  prismatic  layer  is  so  called  because  of  the  colum- 
nar or  prismatic  arrangement  of  the  substance.     The  prisms  stand  perpen- 
dicular to  the  surface.     The  "mother  of  pearl "  is  in  layers  parallel  to  the 


280  CLASSIFICATION    OF   ANIMALS 

surface.  The  mantle  lines  the  entire  inner  surface  of  the  shell.  Some- 
times the  edges  of  the  mantle  are  partly  united.  There  are  always  two 
points,  however,  where  they  are  not  united.  One  is  at  the  posterior  border 
and  one  is  ventral  anterior,  opposite  the  foot.  The  posterior  opening  is 
frequently  double  and  the  edges  of  the  mantle  are  then  often  extended 
so  as  to  form  a  pair  of  tubes  or  siphons,  or  one  double  siphon.  Through 
the  ventral  siphon  the  water  enters  the  mantle  cavity  and  escapes  by  the 
dorsal  siphon. 

606.  The  flattened  body  is  suspended  from  the  dorsal  border  of  the 
mantle  lobes.     At  its  anterior  and  posterior  ends  are  two  strong  muscles 
which  connect  the  two  valves  of  the  shell.     These  are  the  adductors  which 
close  the  shell.     The  gills  are  suspended  from  the  dorsal  border  of  the  body 
in  the  mantle  cavity.     Each  gill  consists  of  two  series  of  parallel  vertical 
bars  or  filaments  which  are  connected  by  short  longitudinal  and  trans- 
verse bars.     The  gill  is  therefore  a  sort  of  double  grating.     On  either 
side  of  the  mouth  are  two  triangular  lappets,  the  labial  palps.     The 
surface  of  the  gills  and  palps  is  covered  with  cilia  which  induce  the  currents 
in  the  water  for  respiration  and  feeding.     The  minute  particles  of  food 
are  carried  toward  the  mouth  along  the  edge  of  the  mantle  and  thence 
between  the  palps.     The  mouth  is  a   simple  opening   into  the    short 
oesophagus.     The  stomach  is  rather  large  and  receives  several  ducts 
from  the  large  digestive  gland.     From  the  stomach  the  intestine  makes 
a  number  of  loops  and  passes  out  of  the  visceral  mass  dorsally  and  pos- 
teriorly to  a  point  above  and  behind  the  posterior  adductor  muscle  into 
the  mantle  cavity. 

607.  The  heart  lies  on  the  dorsal  side  of  the  visceral  mass  and  consists 
of  a  ventricle  and  two  lateral  auricles.     It  is  enclosed  in  a  pericardial 
chamber  which  represents  the  body  cavity.     A  nephridial  funnel  opens 
into  the  pericardial  chamber  on  each  side  and  this  connects  with  the 
kidneys,  or  organs  of  Bojanus.     The  kidneys  open  into  the  mantle  cavity 
on  the  side  of  the  visceral  mass.     The  sexes  are  usually  separate.     The 
gonads  are  large  paired  organs  embedded  in  the  visceral  mass  and  opening 
with  or  near  the  kidney  openings. 

608.  The  nervous  system  consists  of  a  cerebral  ganglion  which  lies 
above  the  oesophagus,  a  visceral  ganglion  below  the  posterior  adductor 
muscle,  and  a  pedal  ganglion  embedded  in  the  foot.     These  ganglia  are 
connected  by  pairs  of  nerves.     Sometimes  there  is  also  a  separate  pleural 
ganglion.     The  special  sense  organs  are  not  well  developed.     There  is  a 
double  statocyst  near  the  pedal  ganglion.     Eyes  are  seldom  found   on 
the  body  but  in  a  number  of  forms  they  occur  on  the  edge  of  the  mantle 


MOLLUSCA 


28l 


and  on  the  siphon.     Tentacles,  or  special  tactile  organs  are  common  on 
the  siphon  and  mantle  edge. 

609.  Fertilization  of  the  eggs  takes  place  in  the  mantle  cavity  where  the 
early  stages  of  development  also  take  place  in  many  cases.     In  the  fresh- 
water   clams    especially,     the 

larvae  remain  for  a  long  time 
attached  to  the  gills  of  the 
parent.  The  marine  forms 
have  a  trochophore  larva.  In 
the  fresh-water  forms  the 
metamorphosis  is  more  com- 
plete and  in  some  cases  the 
larvae  live  for  a  time  as  para- 
sites attached  to  the  gills  and 
fins  of  fishes. 

610.  All  Lamellibranchs  are 
aquatic  and    chiefly     marine. 
Most  live  free  on  the  bottom 
but    some    are    attached    by 
byssus      threads     which     are 
formed  by  the  secretion  of  a 
byssus  gland  in  the  small  foot. 
Others   are   attached    by  the 
cementing  of  one  valve  to  the 
substratum.     Some  bore  into 
wood  and  others  into  calcare- 
ous rocks. 

611.  Order  4. — The    Cepha- 
lopods    are   the    most    highly 
organized  of  all  molluscs  and 
in  some  respects  of  all  inver- 
tebrates.    All  are  marine  and 
some  attain  great  size.     Some 

species  are  known  to  attain,  a      Kingsley.) 

length  of  50-60  feet  including 

the  long  arms.     The  squid,  cuttlefish,  nautilus,  and  octopus  are  some 

of    the    best-known    examples.      Except    in    the    pearly    nautilus     the 

shell  is  always  rudimentary,  and  completely  overgrown  by  the  mantle. 

The  visceral  mass  is   elongated,  conical  in  form,  and  lies   in  a  much 

larger  mantle  chamber.     There  are  two  or  four  plume-like  gills.     The 


FIG.  1 66. — The  "soft  shell"  clarh,  Mya 
arenaria.  Showing  the  position  when 
buried  in  the  mud  with  the  siphons  extend- 
ing to  the  surface.  (From  Galloway  after 


282 


CLASSIFICATION   OF    ANIMALS 


mantle  is  very  thick  and  muscular.  The  foot  is  shaped  like  a  funnel  and 
projects  somewhat  beyond  the  edge  of  the  mantle.  By  the  strong  con- 
traction of  the  mantle  a  stream  of  water  is  shot  out  through  the  funnel 
which  causes  a  backward  movement  of  the  animal.  Less  vigorous  con- 
tractions of  the  mantle  produce  respiratory  currents.  The  large  head  is 
produced  into  eight  or  ten  long  arms  which  encircle  the  mouth.  The  oral 
surface  of  the  arms  is  covered  with  numerous  suckers  which  are  purely 
hold-fast  organs.  The  mouth  opens  into  a  buccal  cavity  and  is  provided 
with  two  strong  jaws  which  together  have  the  form  of  a  beak.  The  buccal 
cavity  contains  a  radula,  and  into  it  open  four  large  salivary  glands.  The 


FIG.  167.— The  Devil-fish  (Octopus),  a  dibranch  Cephalopod.     A,  At  rest;  B, 
swimming,     a,  Arms;  e,  eye;  s,  siphon.     (From  Galloway  after  Merculiano.) 

long  oesophagus  is  sometimes  enlarged  into  a  crop.  The  stomach  consists 
of  two  sacks  into  one  of  which  two  large  digestive  glands  open.  A  large 
gland  secreting  ink  opens  into  the  rectum  near  the  vent.  The  vent  opens 
into  the  mantle  cavity.  The  ink  is  discharged  when  the  animal  is  pursued 
and  serves  to  cover  its  flight.  The  heart  lies  in  the  upper  side  of  the 
visceral  mass.  It  consists  of  a  ventricle  and  as  many  auricles  as  there  are 
gills,  2  or  4.  The  arteries  entering  the  gills  are  enlarged,  muscular  and 
rhythmically  contractile.  They  are  called  branchial  hearts.  There  are 
one  or  two  pairs  of  kidneys  intimately  connected  with  the  circulatory 
system  and  also  connected  with  the  body  cavity  by  nephridial  funnels. 
The  kidneys  open  into  the  mantle  cavity. 


MOLLUSC  A  283 

612.  The  cerebral,  visceral,  pedal,  pleural  and  buccal  ganglia  are  all 
grouped  in  the  region  of  the  buccal  mass.     Other  ganglia  occur  at  the  bases 
of  the  arms  and  in  other  parts  of  the  body.     The  sense  organs  consist  of 
a  pair  of  eyes,  a  pair  of  statocysts  and  a  pair  of  chemical  sense  organs  below 
the  eyes.     The  eyes  are  usually  very  highly  developed  and  have  a  remark- 
able resemblance   to   the  vertebrate   eye,   though   fundamentally  very 
different. 

613.  An  internal  skeleton  of  cartilage  supports  and  protects  the  eyes 
and  central  nervous  system.     Other  cartilages  are  found  at  the  bases  of 
the  arms,  at  the  edge  of  the  mantle  and  in  the  funnel  and  in  the  fin.     The 
rudimentary  shell  in  most  cases  serves  as  a  supporting  structure.     It 
may  be  either  horny  or  calcareous. 


FIG.  168. — The  pearly  Nautilus,  a  tetrabranch  Cephalopod.  e,  Eye;  h,  hood; 
s,  siphon;  se,  septa  forming  the  chambers  of  the  shell;  sp,  siphuncle;  t,  tentacles. 
(From  Galloway  after  Nicholson.) 


614.  The  sexes  are  separate  and  dimorphic.     The  single  gonad  lies  in 
the  end  of  the  visceral  sack  and  its  products  are  emptied  into  the  ccelomic 
cavity.     A  pair  of  complicated  ducts  with  associated  glands  lead  from  the 
body  cavity  to  the  mantle  cavity.     One  of  these  ducts  is  frequently 
rudimentary  or  wanting.     Development  is  direct. 

615.  The  cephalopods  are  carnivorous,  using  their  arms  for  catching 
their  prey.     When  on  the  bottom  the  arms  are  also  used  for  locomotion. 
By  means  of  the  mechanism  already  described  a  strong  swimming  stroke 
which  carries   the  animal  backward  is  performed.     Some  species  are 
habitually  swimming,  others  keep  close  to  the  bottom. 


284  CLASSIFICATION   OF    ANIMALS 

6 1 6.  The  sub-order  Tetrabranchiata  is  characterized  by  the  four  gills, 
numerous  tentacles  in  place  of  the  arms  and  a  large  many-chambered 
shell  of  which  only  the  last,  largest  chamber  is  occupied  by  the  animal. 
The  ink  bag  is  wanting.     There  is  only  one  living  species,  the  pearly 
Nautilus.     The  Dibranchiata  have  only  two  gills  and  a  rudimentary  shell. 
In  the  section  Decapoda  there  are  eight  arms  and  two  longer  tentacle 
arms.     The  suckers  are  stalked  and  have  a  horny  rim.     There  are  two 
lateral  fins.     In  the  section  Octopoda  the  tentacles  are  wanting,  the 
suckers  are  sessile  and  without  a  horny  rim.     There  are  usually  no  fins. 

Cephalopoda : 

Sub-order.  Tetrabranchiata,  Nautilus. 
Sub-order.  Dibranchiata. 

Section.  Decapoda,  Squid. 

Section.  Octopoda,  Octopus. 

617.  PHYLUM  IX. — Adelochorda. 

6 1 8.  Class  I.    Enteropneusta. — This  phylum  and  class  are 
represented  by  a  few  genera  of  worm-like  animals  which  are  of 
interest  because  they  form  one  of  the  links  connecting  the  in- 
vertebrates and  vertebrates.     A  representative  of  the  group 
common   on   our  Atlantic   seashore   is   Dolichoglossus.     The 
animal  burrows  in  the  sand  and  mud  along  shore.     When  the 
tide  is  out  the  coiled  castings  of  this  animal  are  often  seen  form- 
ing piles  several  inches  in  height.     The  coiled  castings  and  an 
odor  of  iodoform  are  indications  of  Dolichoglossus.     The  body 
is  composed  of  a  conical  proboscis,  a  broad  band-like  " collar" 
and  a  long  tapering  trunk.     Only  three  points  in  the  anatomy 
need  be  mentioned,     i.  The  mouth  lies  in  front  of  the  collar 
and  from  here  the  digestive  tract  extends  directly  to  the  poste- 
rior end  of  the  body.     The  anterior  part  of  the  digestive  tract 
is   differentiated  for  respiration.     It  is  connected  at  regular 
intervals  with  the  body- wall  and  at  these  points  there  are  open- 
ings which  form  a  passage  from  the  enteric  cavity  to  the  ex- 
terior.    These  openings  are  called  gill  slits.     The  respiratory 
current  enters  the  mouth  and  passes  out  through  these  slits. 
2.  The  nervous  system  consists  of  two  ganglionic  chains,  one 


ADELOCHORDA  285 

dorsal  and  one  ventral.  These  are  connected  by  a  nerve  ring 
in  the  region  of  the  collar.  The  dorsal  nerve  chain  is  tubular 
in  front  of  the  nerve  ring.  3.  The  dorsal  wall  of  the  digestive 
tract  is  prolonged  forward  into  the  proboscis  as  a  stiff  tube  of 
cells  which  forms  a  supporting  axis  for  the  proboscis.  Neither 
of  these  features  are  found  in  any  of  the  phyla  so  far  described 
but  they  are  regarded  as  the  homologues  of  the  pharyngeal 
gill  slits,  dorsal  tubular  nervous  system,  and  notochord,  re- 
spectively, of  the  Vertebrates.  The  validity  of  the  third 
homology  may  be  seriously  questioned. 

619.  PHYLUM  X.     Urochorda. — The  Urochorda  are  also  called 
Tunicata  because  of  the  tunic  or  test,  a  thick  integumentary 
structure  formed  by  the  mantle  in  many  forms.     This  test 
is  remarkable  because  it  contains  cellulose,  which  is  otherwise 
found  only  in  plants.     The  test  is  sometimes  gelatinous  but 
is  often  extremely  tough  and  resistant.     Many  Tunicata  are 
fixed  but  there  are  also  free  swimming  forms.     In  the  adult, 
the  animals  are  usually  markedly  degenerate.     The  body  is 
often  sack-like  in  form.     There  is  a  large  pharynx  with  gill 
slits,  a  dorsal  tubular  nervous  system  and  a  notochord.     The 
food  in  minute  particles  is  collected  from  the  respiratory  current 
and  directed  to  the  oesophagus  by  the  action  of  ciliated  grooves 
in  the  pharynx.     The  Tunicata  are  all  marine. 

620.  Class  I.     Copelata. — This  class  comprises  free  swim- 
ming forms  in  which   the   notochord   persists  in   the   adult. 
The  gill  slits  open  directly  to  the  exterior.     The  body  is  cask- 
shaped  and  there  is  a  flat  tail.     The  mantle  is  readily  cast  off 
and  reformed. 

621.  Class   II.    Tethyodea. — The  Ascidians  or  sea  squirts 
are  for  the  most  part  fixed.     The  gill  slits  and  vent  open  into 
a  chamber,   "atrium,"   formed  by  folds  of  the  integument. 
The  atrial  opening  is  usually  near  the  mouth.     Both  mouth 
and  atrial  opening  can  be  closed  by  muscular  contraction.     The 
whole  body  can  also  be  greatly  contracted.     From  the  pharyn- 


286  CLASSIFICATION   OF   ANIMALS 

geal  chamber  a  short  oesophagus  leads  to  a  stomach  into  which 
the  ducts  of  a  digestive  gland  open.  There  is  then  a  short 
coiled  intestine  which  opens  into  the  atrium.  There  is  a  heart 
but  the  vascular  system  is  not  well  developed.  The  nervous 
system  consists  of  a  single  ganglion  lying  between  the  mouth 
and  atriopore.  The  sense  organs  are  not  well  developed.  All 
Tunicata  are  hermaphroditic.  The  larva  develops  into  a 
"tadpole"  which  shows  marked  vertebrate  affinities.  Extend- 
ing through  the  tail  and  for  some  distance  into  the  body  of  the 
"tadpole"  is  a  rod  of  large  cells  which  forms  a  supporting  axis. 
This  is  the  notochord.  On  its  dorsal  side  is  a  long  tubular 
nervous  system  which  ends  in  front  in  a  vesicle  containing  an 
eye  and  a  statocyst.  At  the  anterior  end  of  the  body  there  are 
three  glandular  papillae  by  which  the  tadpole  finally  attaches 
itself.  A  metamorphosis  then  takes  place  in  which  the  chorda 
entirely  disappears  and  the  nervous  system  is  reduced  to  the 
single  ganglion.  The  entire  tail  is  resorbed  and  by  a  twisting 
of  the  body  in  the  further  process  of  development  the  mouth 
comes  to  lie  opposite  the  point  of  attachment. 

622.  Reproduction  also  takes  place  asexually  and  in  many 
forms  colonies  are  formed  by  budding.     The  colonies  may  be 
free  swimming. 

623.  Class  III.    Thaliacea. — The  Thaliacea  are  free  swim- 
ming, transparent,  colonial  forms  with  an  alternation  of  gen- 
erations.    Solitary  individuals  give  rise  to  a  colony  of  sexual 
individuals  by  budding  and  the  colonial  individuals  produce 
eggs  which  develop  into  the  sexless  solitary  form. 

624.  PHYLUM  XI.     Acrania. — This  phylum  has  so  much  in 
common  with  the  vertebrates  that  the  absence  of  a  skull  was 
considered  of  sufficient  note  to  be  indicated  in  the  name  of 
the  phylum.     The  body  is  elongated,  flattened  laterally  and 
pointed  at  both  ends.     There  is  a  persistent  notochord,  a  long 
series  of  gill  slits  and  a  dorsal  tubular,  nervous  system.     The 
body  is  segmented  but  there  are  no  paired  appendages. 


ACRANIA  287 

625.  Class  Leptocardia. — There  is  only  one  class  represented 
by  a  few   species.     The  form  of  the  body  has  given  rise  to 
the  common  name  "lancelet."     The  integument  of  the  lancelet 
consists  of  a  single  layered  epidermis.     The  gill  slits  open  into 
a  peribranchial  chamber  formed  by  folds  of  the   skin.     This 
chamber  opens   to   the  exterior  by  a  pore  at  its  posterior  end. 
The  mouth  is  surrounded  by  a  circle  of  cirri.     The  animal  lies 
on  its  side  partly  buried  in  the  sand  and  collects  its  food  from 
the  respiratory  current  which  is  produced  by  ciliary  action. 
The  intestine  is  a  straight  tube  opening  by  a  vent  on  the  left 
side  of  the  tail.     A  long  glandular  pocket  connected  with  the 
intestine  probably  represents  a  digestive  gland.     There  is  no 
heart  but   the  larger  vessels   drive   the  blood  by   pulsating 
contraction.     There  is  a  ventral  vessel  (truncus  arteriosus)  which 
carries  the  blood  forward.     From  this  lateral  branches  pass 
over  the  gill  arches  to  unite  above  in  another  vessel  through 
which  the  blood  flows  back  toward  the  posterior  end  of  the  body. 
A  portal  vein  connects  the  intestine  and  digestive  gland  (liver), 
and  other  veins  from  the  body-wall  and  digestive  gland  unite 
in  the  truncus  arteriosus.     In  the  region  of  the  gills  there  is  a 
paired  series  of  nephridia    which  begin  with  funnels   in   the 
ccelomic   cavity   and   open   into   the   peribranchial   chamber. 
The  lancelet  cannot  be   said  to  have  a  brain.     The  anterior 
end  of  the  nerve  tube  (spinal  cord)  is  slightly  enlarged  and  con- 
tains a  vesicular  enlargement  of  the  central  canal.     Connected 
with  this  is  a  single  eyespot  and  an  olfactory  groove.     Numer- 
ous other  eyespots  lie  scattered  along  the  spinal  cord.     The 
gonads  lie  against  the  wall  of  the  peribranchial  chamber  which 
breaks  to  permit  the  reproductive  cells  to  escape.     The  develop- 
ment of  the  lancelet  is  by  a  metamorphosis.     The  early  stages 
are  described  in  Part  III. 

626.  PHYLUM  XII.     Vertebrata. — The  Vertebrates  all  have  a 
series  of  gill  slits.     In  the  terrestrial  forms   these  are  only 
present  in  the  larval  stages.     The  nervous  system  arises  as  a 


288  CLASSIFICATION   OF   ANIMALS 

tubular  infolding  of  the  ectoderm  of  the  dorsal  side.  The 
axial  skeleton  consists  primarily  of  a  notochord  which  persists 
in  the  lower  forms  but  is  only  found  in  the  larval  stages  of  the 
higher  forms.  In  addition  a  vertebral  column  and  a  skull 
supplement  or  replace  the  notochord.  The  vertebral  column 
consists  of  a  series  of  segmentally  arranged  vertebrae  with  dorsal 
arches  protecting  the  nervous  system  and  ventral  arches  pro- 
tecting the  viscera. 

627.  Class  I.  Cyclostomata. — The  round  mouth  eels.  This 
class  comprises  only  a  few  species  of  eel-like  animals  which 
are  destitute  of  a  lower  jaw.  The  skin  is  smooth,  i.  e.,  there 
are  no  scales.  There  are  no  paired  appendages.  There  is  a 
median  dorsal  fin  which  is  continued  around  the  tip  of  the  tail 
forming  a  tail  fin.  The  mouth  forms  a  circular  sucking  disc 
which  is  covered  with  hard  epidermal  tubercles  by  which  the 
animal  bores  through  the  skin  of  the  host  to  which  it  attaches 
itself.  The  Cyclostomes  are  ectoparasites  and  some  even  make 
their  way  for  some  distance  into  the  host.  The  alimentary 
canal  is  practically  a  simple  tube,  though  some  forms  have  a 
spiral  valve;  and  there  is  a  large  liver  which  opens  into  the 
digestive  tract  by  a  duct.  There  are  also  glands  in  the  wall 
of  the  intestine.  There  are  6-14  pairs  of  gill  slits  which  open 
directly  to  the  exterior.  The  skeletal  system  consists  of  a 
well-developed  notochord  with  a  thick  fibrous  sheath,  and  a 
number  of  cartilages.  The  brain  is  enclosed  in  a  skull  composed 
partly  of  cartilage,  partly  of  membrane.  To  this  are  attached 
the  two  cartilaginous  ear  capsules  and  a  cartilaginous  nasal 
capsule.  There  is  also  a  network  of  cartilages  surrounding  and 
supporting  the  mouth  and  pharyngeal  regions.  The  vertebrae 
consist  only  of  neural  arches  with  intercalary  pieces,  and  of 
haemal  arches  in  the  tail  region.  There  is  a  heart  similar  to 
that  of  fishes.  The  olfactory  organ  is  a  single  sack  with  a 
median  opening.  The  eyes  are  of  the  typical  vertebrate  type 
but  in  some  cases  more  or  less  reduced.  The  ear  is  a  simple 


CYCLOSTOMATA  289 

statocyst-like  sack.  The  brain  is  well  developed,  but  lacks  cere- 
bral hemispheres  and  cerebellum.  The  ten  cranial  nerves  are 
of  a  simple  type  and  the  spinal  nerves  do  not  unite  dorsal  and 
ventral  roots. 

628.  Class  II.    Pisces. — The  Fishes  are  a  large  group  in  which 
are  included  animals  of  very  diverse  character.     The  skin  is 


FIG.  169. — The  Devil-ray,  Manta.  The  lateral  expansions  are  developed 
from  the  pectoral  fins.  This  is  a  ventral  view  and  shows  the  five  pairs  of  gill 
slits,  the  odd  shaped  head  with  the  eyes  on  the  sides  of  the  horns,  the  wide, 
straight  mouth,  and  the  whip-like  tail.  Taken  at  Beaufort,  N.  C.  Much 
reduced. 

usually  covered  with  bony  scales.  There  is  a  median  fin  which 
may  be  divided  into  several  parts.  There  may  be  one  or  more 
dorsal  fins,  a  tail  fin  and  a  ventral  fin.  There  are  usually 
two  pairs  of  appendages  which  also  have  the  form  of  fins.  The 
19 


2 QO  CLASSIFICATION   OF  ANIMALS 

fins  are  all  dermal  expansions  supported  on  cartilaginous  rays 
and  bony  spines.  Fishes  are  all  aquatic  and  respiration  is  by 
means  of  gills.  There  are  often  accessory  respiratory  organs — 
the  swim  bladder  and  true  lungs.  The  swim  bladder  is  an 
unpaired  sack  filled  with  air  which  serves  to  give  the  body  of 
the  fish  the  same  specific  gravity  as  the  water.  The  heart 
consists  of  three  chambers,  a  thin- walled  auricle  opening  ante- 
riorly into  a  very  strong  thick- walled  ventricle  which  in  turn  is 
continued  anteriorly  by  the  conus  arteriosus.  In  the  bony 
fishes  the  latter  is  wanting  and  in  its  place  the  truncus  arteriosus 
is  enlarged  into  a  bulbus  arteriosus.  The  vessels  have  prac- 
tically the  same  arrangement  as  in  the  lancelet.  The  organs  of 
special  sense  are  two  nasal  chambers  which  do  not  communicate 
with  the  mouth  but  have  each  two  openings  on  the  surface,  one 
incurrent,  one  excurrent  orifice;  two  eyes;  two  statocysts  with 
utriculus  and  sacculus  and  three  semicircular  canals;  and  the 
lateral  line  system.  The  lateral  line  organs  line  depressions  or 
tubes  which  communicate  with  the  surface.  These  organs  are 
arranged  in  a  line  along  each  side  of  the  body  and  other  shorter 
lines  along  the  side  and  over  the  dorsal  surface  of  the  head. 
The  sensory  cells  are  clustered  and  their  sensory  bristles 
project  into  the  canal  of  the  lateral  line.  The  function  is  to 
sense  the  currents  in  the  water.  The  cerebral  hemispheres  are 
small,  the  cerebellum  comparatively  well  developed. 


629.  Order  i. — The  Selachii  are  the  sharks  and  rays;  fishes  with  a 
cartilaginous  skeleton,  placoid  scales,  5-7  gill  clefts  with  separate  openings, 
a  spiral  valve,  and  upper  jaw  not  united  with  the  skull.     The  body  is 
spindle  shaped  in  the  sharks  and  flattened  dorso-ventrally  in  the  rays. 
The  spiral  valve  is  a  much  enlarged  posterior  portion  of  the  intestine  with 
a  spiral  shelf-like  fold  projecting  inward  from  the  wall.     These  fishes  are 
chiefly  marine. 

630.  Order  2. — The  Holocephali  are  not  numerous  in  point  of  genera. 
They^have  a  cartilaginous  skeleton  with  the  upper  jaw  articulated  with 
the  skull.     The  notochord  persists  and  the  vertebrae  are  represented  only 


PISCES  291 

by  thin  calcareous  rings  in  the  chorda  membrane.     There  is  only  one 
pair  of  external  openings  of  the  gill  clefts. 

631.  Order  3. — The  Dipnoi  or  lung  fishes.     In  this  group  there  are  four 
pairs  of  gills  which  are  covered  by  an  operculum.     The  tail  is  diphycercal 
i.  e.,  the  tail  fin  is  symmetrical  around  the  straight  spinal  axis.     The 
chorda  persists.     The  skeleton  is  cartilaginous  and  partly  bony.     There 
is  a  spiral  valve  and  a  pair  of  lungs.     These  fishes  live  in  tropical  regions 
in  rivers  and  ponds  which  dry  up  in  the  dry  season.     During  the  dry  season 
the  animals  bury  themselves  in  the  mud. 

632.  Order  4. — The  Brachioganoidea    also  have  gills  covered  by  an 
operculum  and  a  diphycercal  tail.     The  skeleton  is  bony.     The  body  is 
covered  with  thick  rhombic  scales  covered  with  ganoin,  a  kind  of  enamel. 
There  is  also  a  spiral  valve. 

633.  Order  5. — The  Chondroganoidea  or  sturgeons  have  the  gills  covered 
by  an  operculum;  a  persistent  chorda  and  cartilaginous  skeleton;  the  head 
prolonged  into  a  snout;  the  skin  is  naked  or  with  bony  plates;  the  tail 
fin  heterocercal,  i.  e.,  the  axis  is  bent  up  and  the  fin  is  unsymmetrical; 
a  spiral  valve  and  a  conus  arteriosus.     The  number  of  genera  is  small. 
The  fishes  are  found  chiefly  in  fresh  waters. 

634.  Order  6. — The  Rhomb oganoidea  or  gar  pikes  have  gills  covered  by 
an  operculum;  a  bony  skeleton;  a  long  snout;  body  covered  with  rhombic 
ganoid  scales;  tafl  heterocercal;  rudimentary  spiral  valVe  and  a  conus 
arteriosus.     A  few  species  only;  found  in  the  rivers  and  lakes  of  North 
America. 

635.  Order   7. — The   Cycloganoidea   have   operculum,   bony   skeleton, 
cycloid  scales,  tail  heterocercal,  rudimentary  spiral  valve  and  a  conus 
arteriosus.     There  is  only  one  species,  Amiatus  calvus.     This  is  found  in 
the  streams  of  North  America. 

636.  Order  8.— The  Teleostei  or  true  bony  fishes  are  very  numerous. 
They   have   an  operculum,  bony  skeleton,  ctenoid  or  cycloid  scales  or 
large  bony  plates,  a  bulbus  arteriosus,  no  spiral  valve.     The  order  is 
divisible  into  twelve  sub-orders  with  many  families.     Some  of  the  families 
are  the  herrings,  salmon,  electric  eel,  carp,  catfish,  eels,  pike,  sea  horse, 
mullet,  perch,  mackerel,  flat  fishes,  toad  fish,  trunk  fish,  etc. 

637.  Class  HI.    Amphibia. — In  this  group  of  animals  the 
larva  is  typically  aquatic,  the  adult  terrestrial.     This  is  pri- 
marily evident  in  the  respiratory  organs  though  in  many  cases 
a  marked  metamorphosis  occurs  which  involves  other  organs. 
The  Amphibia  have  two  pairs  of  pentadactyl  appendages,  a 


CLASSIFICATION  OP  ANIMALS 


ft.-  — 


U-d  el. 


— ft. 


FIG.  170. — Diagrams  of  the  girdles  and  appendages  of  a  typical  Vertebrate. 
A,  Anterior;  B,  posterior;  ac,  acetabulum;  c,  coracoid;  ca,  carpals;  ce,  centralia; 
d.c.,  distal  carpals;  d.t.,  distal  tarsals;  el,  elbow-joint;  /,  fibula;  fe,  femur;  fi, 
fibulare;  g.c.,  glenoid  cavity;  h,  humerus;  il,  ilium;  in,  intermediale;  is,  ischium; 
kn,  knee-joint;  m.c.,  metacarpals  (1-5);  m.t.,  metatarsals  (1-5);  p  pubis;  ph, 
phalanges  (1-5);  pr.c.,  pre-coracoid;  r,  radius;  ra,  radiale;  sc,  scapula;  t,  tibia; 
ta,  tarsals;  ti,  tibiale;  u,  ulna;  ul,  ulnare.  (From  Galloway.) 


AMPHIBIA 


293 


naked  skin,  gills  in  the  larval  stages,  lungs  in  the  adult,  a  three- 
chambered  heart  of  one  ventricle  and  two  auricles.  The  larva 
is  a  tadpole  with  a  broad  tail  but  no  paired  appendages.  It  is  a 
vegetable  feeder  and  has  a  long  coiled  intestine.  In  meta- 
morphosis the  tail. shrivels  as  the  legs  develop.  At  the  same  time 
the  gills  are  also  resorbed  and  the  lungs  are  developed  and  be- 


FIG.    171. — Salamandra   maculosa,   the   fire   salamander   of  Europe.     Slightly 

reduced. 

come  functional  as  respiratory  organs.  The  adult  is  carnivorous 
and  the  digestive  tract  is  relatively  shorter  in  correspondence 
to  the  character  of  the  diet.  This  is  the  type  of  metamorphosis 
which  occurs  in  the  frogs  and  toads.  The  newts  and  salaman- 
ders undergo  a  less  radical  transformation.  The  skeleton  is 


294 


CLASSIFICATION   OF   ANIMALS 


bony,  though  parts  remain  cartilaginous  in  the  adult.  The 
skeleton  is  so  much  like  that  of  the  higher  vertebrates  that 
most  parts  can  be  accurately  homologized.  The  same  is  true 
of  the  digestive  tract.  There  is  a  cloaca  into  which  the  intes- 
tine, the  ureters  and  the  genital  ducts  open.  On  its  ventral 
wall  there  is  a  large  pocket,  the  urinary  bladder.  The  lungs  are 
simple  sacks  with  the  walls  usually  more  or  less  folded  like  a 
honey  comb.  The  thin  skin  also  acts  as  a  respiratory  organ. 


FIG.  172. — Outline  drawings  of  three  urodele  amphibians  showing  successive 
stages  in  degeneration  of  the  appendages.  A,  Siren;  B,  Amphiuma;  C,  Necturus. 
(From  Galloway,  after  Mivart.) 

The  kidneys  open  into  the  cloaca  by  a  pair  of  ureters.  The 
oviducts  are  two  long  convoluted  tubes  beginning  in  the  anterior 
part  of  the  body  cavity  by  large  funnel-like  openings  and  lead- 
ing separately  into  the  cloaca.  The  male  gonads  are  connected 
with  the  kidneys  and  the  sperm  reaches  the  cloaca  by  way  of  the 
urinary  tubules.  The  nostrils  have  an  opening  into  the  anterior 
part  of  the  mouth.  The  brain  is  well  developed  but  the  cere- 
bellum is  small. 

638.  Order  i. — The  Gymnophiona  are  worm-like  Amphibia,  without 
appendages.  The  trunk  is  elongated  and  the  tail  rudimentary.  The 
skin  is  filled  with  small  scales.  The  chorda  is  persistent.  The  eyes  are 


AMPHIBIA 


295 


not  well  developed  and  the  ear  drum  and  middle  ear  are  wanting.     This 
is  a  small  group,  found  in  tropical  regions  living  underground. 

639.  Order  2. — The  Urodela  have  an  elongated  body  with  a  tail  and 
usually  weak  legs.  In  one  family  (Sirenida?)  the  posterior  legs  are 
wanting.  In  some  families  the  gills  are  retained  in  the  adult  and  in 
others  the  gills  are  lost  but  the  gill  slits  are  retained.  In  most  cases,  how- 
ever, the  gills  and  slit  both  disappear.  The  eyes  are  small  and  the  ear 


g-w. 


FIG.  173. — Diagram  of  a  bird  embryo  within  the  egg  membrane.  The  foetal 
membranes  are  omitted  (see  next  figure),  b,  Brain;  b.w.,  body  wall;  c.c,  central 
canal  of  spinal  cord;  co,  ccelom;  g,  intestine;  g.w.,  wall  of  intestine;  s.c.,  spinal 
cord;  y.s.,  yolk-sac. 

drum  and  middle  ear  are  absent,       To  this  order  belong  newts,  efts, 
"spring  lizards,"  "mud  puppies"  and  salamanders. 

640.  Order  3. — The  Anura  comprise  frogs,  toads  and  tree  toads.     The 
body  is  short  and  tailless.     The  posterior  pair  of  appendages  are  usually 
long  and  strong.     The  eyes  are  large;  there  is  usually  an  ear  drum  with 
a  middle  ear  communicating  with  the  mouth  cavity. 

641.  Class  IV.    Reptilia. — The  Rep  tiles  a  re  typically  terres- 
trial though  many  live  in  the  water.     They  never  have  gills. 
The  skin  is  covered  with  horny  scales  or  plates  formed  by  the 


296 


CLASSIFICATION   OF   ANIMALS 


epidermis.  There  are  typically  two  pairs  of  appendages  but 
these  have' been  lost  in  many  of  the  Squamata.  The  heart  has 
two  auricles  and  the  ventricle  is  partly  divided  by  an  incom- 
plete partition.  The  lungs  are  spongy  in  structure.  There 
are  twelve  cranial  nerves.  This  comprises  the  ten  cranial 
nerves  of  the  amphibia  and  the  first  spinal  nerve  as  well  as  a 
spinal  accessory  nerve  not  represented  as  a  separate  nerve  in 


arn.c 


CO. 


K 

ff 

-  a  ;»"* 
-co. 

FIG.  174. — Diagram  of  a  bird  embryo  with  the  foetal  membranes,  the  amnion 
and  the  allantois.  am1,  inner  or  true  amnion;  aw2,  outer  or  false  amnion;  am.c, 
amniotic  cavity;  al,  allantois.  Other  lettering  as  in  the  preceding  figure. 

Amphibia.  The  cerebral  hemispheres  are  well  developed.  The 
intestine,  ureters  and  genital  ducts  open  into  a  cloaca.  The 
eggs  are  fertilized  in  the  oviduct.  They  are  very  large  and  are 
covered  by  a  tough  shell  secreted  by  the  oviduct.  The  eggs 
are  generally  not  brooded  but  are  buried  in  the  earth  and  allowed 
to  develop  at  atmospheric  temperature.  The  embryo  is  pro- 
vided with  the  fcetal  membranes,  the  amnion  and  allantois. 


REPTILIA  297 

642.  Order   i, — Rhynchocephalia  are   represented   by   a  single  species 
living  on  islands  off  the  coast  of  New  Zealand.     The  animals  are  lizard- 
like,  but  more  primitive  in  a  number  of  ways. 

643.  Order  2. — The  Testudinata  or  turtles  have  a  very  compact  form 
with  bony  dorsal  and  ventral  shields,  the  carapace  and  plastron.     The 
jaws  are  covered  with  a  horny  sheath  forming  a  beak.     Teeth  are  wanting. 
The  carapace  is  formed  by  the  broad  dorsal  spines  and  the   much   ex- 
panded ribs  together  with  a  series  of  marginal  plates  of  dermal  bone. 
The  plastron  is  chiefly  composed  of  plates  of  dermal  bone.     The  shell  is 
covered   with   thick   horny   scales,    the   "tortoise   shell."     Most  of  the 
Testudinata  are  aquatic.     Snapping  turtle,  terrapin,  tortoise,  and  sea 
turtles. 

644.  Order  3. — The  Emydosauria  are  large  aquatic  lizard-like  reptiles. 
The  alligator,  crocodile  and  gavial  are  well  known.     There  are  only  a 
few  genera.     The  skin  contains  bony  plates  as  well  as  horny  scales.     The 
teeth  are  set  in  sockets.     The  ventricle  is  completely  divided  into  two 
chambers. 

645.  Order    4. — The    Squdmata    comprise    both    lizards    and    snakes. 
Usually  the  lizards  have  two  pairs  of  appendages  while  the  snakes  have 
none,  but  among  the  lizards  are  found  various  stages  of  degeneration  of 
the  appendages  even  to  forms  in  which  no  evidence  of  limbs  is  discernible 
externally.     On  the  other  hand  among  snakes  rudiments  of  appendages 
are  also  found.     The  Squamata  are  distinguished  from  the  other  reptile 
orders  by  the  movable  quadrate  bone.     In  the  sub-order  Lacertilia,  the 
lizards,  the  upper  jaws  are  not  movable.     The  tongue  is  flat.     There  is 
a  urinary  bladder.     In  the  sub-order  Ophidia,  the  upper  jaw  is  movable, 
the  tongue  is  forked  and  enclosed  in  a  sheath  and  there  is  no  bladder. 
The  ear  drum  and  middle  ear  are  also  wanting.     Another  small  sub- 
order of  lizard-like  forms,  including  the  chameleon,  are  tree  dwellers  and 
as  a  special  adaptation  to  such  conditions  the  toes  are  opposable  for  clasp- 
ing, and  the  tail  is  prehensile. 

646.  Class  V.    Aves. — The  Birds  are  in  many  respects  the 
most   highly  specialized  of  all  animals.     The  feathers,  which 
are  characteristic  of  the  class,  are  specialized  epidermal  struc- 
tures and  are  very  remarkable.     The  skin  is  comparatively  thin 
but  the  feathers  more  than  compensate  as  protective  structures. 
For  resistance  to  mechanical  injury,  or  protection  from  cold, 
or  heat,  or  wetting,  or  adaptation  to  thermal  control,  or  for  the 


298  CLASSIFICATION   OF   ANIMALS 

possibilities  of  ornamentation  either  in  form  or  color,  it  is  diffi- 
cult to  find  within  the  entire  range  of  the  animal  kingdom  more 
efficient  structures.  In  their  ability  to  fly  we  have  another  evi- 
dence of  high  specialization.  Both  pairs  of  appendages  are 
fundamentally  pentadactyl  but  the  anterior  pair  has  undergone 
a  profound  modification.  The  bones  of  the  upper  arm  and  fore 
arm  are  of  the  typical  pentadactyl  type  but  the  hand  is  reduced 
to  the  three  matacarpals  of  the  ist,  2nd,  and  3rd  digits  and  two, 
three,  and  one  phalanges  respectively.  The  muscular  develop- 
ment is  concentrated  in  the  muscles  which  move  the  wing  as  a 
whole,  viz.,  those  connecting  the  wing  with  the  sternum.  This 
requires  a  great  development  of  the  surface  of  the  sternum  and 
its  keel  to  which  these  muscles  are  attached.  The  other  mus- 
cles of  the  wing  are  greatly  reduced.  Another  anatomical 
peculiarity  which  is  thought  to  be  an  adaptation  to  flight  is 
the  comparatively  small  head  of  the  bird.  This  is  due  chiefly 
to  the  absence  of  the  organs  for  mastication,  teeth,  heavy  upper 
and  lower  jaws  and  heavy  masseter  muscles.  The  absence  of 
these  organs  is  compensated  for  by  the  crop  in  which  the  food 
is  softened,  and  the  gizzard  in  which  it  is  triturated.  By  this 
substitution  the  weight  of  the  body  is  concentrated  and  the 
centre  of  gravity  lowered.  The  posterior  appendages  are  also 
peculiar.  The  pelvic  girdle  is  attached  to  at  least  six  vertebrae 
but  is  open  below,  that  is,  there  is  no  symphisis  pubis.  This 
condition  of  the  pelvic  girdle  allows  of  the  passage  of  the  rela- 
tively very  large  eggs  of  the  bird.  The  fibula  is  rudimentary 
and  the  proximal  tarsals  are  fused  with  the  tibia  forming  a 
single  bone,  the  tibio-tarsus.  The  distal  tarsals  are  fused  with 
the  metatarsals  to  form  a  tarso-metatarsal.  The  fifth,  and 
sometimes  the  first,  digits  are  wanting. 

647.  The  heart  is  completely  divided  into  four  chambers  as  in 
the  Crocodilia  and  Mammals  but  the  blood  from  the  left  ventricle 
goes  to  the  lungs  while  that  from  the  right  goes  to  the  general 
systemic  circulation,  reversing  the  order  as  found  in  Mammals. 


AVES  299 

The  lungs  are  connected  with  an  extensive  system  of  air  spaces 
which  penetrate  far  into  other  parts  of  the  body,  even  penetra- 
ting the  bones  and  replacing  the  marrow.  The  vocal  cords 
are  located  at  the  junction  of  the  bronchi  in  an  organ,  the  syrinx, 
which  takes  the  place  of  the  larynx.  The  right  ovary  is  want- 
ing and  the  corresponding  oviduct  is  rudimentary.  The  eggs 
are  fertilized  in  the  upper  part  of  the  oviduct  and  are  then 
surrounded  by  layers  of  albumen,  membraneous  shell  and  cal- 
careous shell  in  succession,  as  they  pass  down  the  oviduct. 

648.  The  eyes  and  ears  are  highly  developed.     There  are  two 
eyelids  and  a  nictitating   membrane.     The   ear  is  without   a 
concha.     The  brain  shows  a  considerable  advance  over  that  of 
Reptiles,  especially  with  regard  to  the  development  of  cerebrum, 
optic  lobes  and  cerebellum. 

649.  In  many  points  Birds  differ  radically  from  Mammals 
and  at  the  same  time  show  a  strong  resemblance  to  Reptiles. 

650.  In  a  comparatively  small  group  of  birds  the  wings  are  not  used 
and  are  consequently  rudimentary.     These  are  the  running    birds    or 
Ratitae — ostrich,  emeu  and  cassowary,  the  almost  extinct  apteryx  of  New 
Zealand  and  the  recently  extinct  moa  of  New  Zealand.     With  the  disuse 
of  the  wing  the  muscles  have,  degenerated  and  with  them  the  keel  of  the 
breast  bone,  their  point  of  attachment.     Other  birds  are  called  Carinatae 
because  of  the  keel  of  the  breast  bone.     They  are  divided  into  seventeen 
orders  as  indicated  in  the  following  synopsis. 

Ratitae. 

1.  Struthiomorphce.     Ostrich,  rhea,  cassowary. 

2.  Dinornithes.     The  recently  extinct  gigantic  Dinornis. 

3.  Aepyornithes.     The  recently  extinct  yEpyornis. 

4.  Apteryges.     The  Kiwi  Kiwis  of  Australia  and  New  Zealand. 
Carinatae. 

5.  Tinamiformes.     South  American  fowl-like  birds. 

6.  Gallinacei.     Pheasants,  turkey,  fowl,  quail. 

7.  Columba.     Pigeons,  doves.     The  extinct  dodo. 

8.  Lari.     Gulls. 

9.  GrallcE.     Rails,  cranes. 

10.  Lamellirosires.     Geese,  ducks,  flamingo. 

11.  Ciconice.     Ibis,  storks,  herons. 


300 


CLASSIFICATION   OF   ANIMALS 


12.  Steganopodes.     Pelican,  frigate  bird. 

13.  Tubinqres,     Stormy  petrel,  albatross. 

14.  Impennes.     Penguin. 


FIG.  175. — The  kiwi,  Apteryx.     Xi/4-     The  outline  in  the  background 
represents  the  size  of  the  wing. 

15.  Pygopodes.     Divers,  grebes. 

1 6.  Accipitres.     Condor,  vultures,  eagles,  hawks,  falcons. 

17.  Striges.     Owls. 


AVES  301 

1 8.  Psittad.     Parrots. 

19.  Coccygomorpha.     Cuckoo. 

20.  Pid.     Woodpeckers. 

21.  Cypselomorpha.     Whip-poor-will,  bull  bat,  humming  birds. 

22.  Passer es.     The  song  birds;  a  very  large  order. 

Sub-order  i.  Clamatores.     King  bird. 

Sub-order  2.  Oscines.  Swallows,  fly  catchers,  warblers,  thrushes, 
black  birds,  mocking  bird,  cat  bird,  larks,  titmouse,  crows,  ravens, 
finches,  sparrows. 

651.  Class    VI.     Mammalia. — The  Mammals  are  typically 
covered  with  hair.     In  several  cases  the  hairs  are  scattered  or 
limited  to  the  " whiskers"  as,  e.  g.,  in  some  marine  mammals — 
the  sea  cow  and  whale.     The  function  of  the  hair  is  primarily 
to  retain  the  heat  of  the  body.     When  the  hair  is  wanting  the 
function  may  be  performed  by  a  thick  layer  of  fat  beneath  the 
skin.     The    name  mammal  refers  to  the  mammary  glands  in 
the  skin  of  the  ventral  surface  of  the  body.     There  are  usually 
two  sets  of  teeth,  first  a  milk  dentition  which  is  later  replaced 
by  a  permannt  dentition.     The  latter  consists  of  four  kinds, 
incisors,  canines,  pre-molars  and  molars.     The  heart  consists 
of  four  chambers;  the   red  blood    corpuscles   are   without   a 
nucleus;  the  temperature  of  the  body  is  constant.     The  lungs 
and  heart  are  separated  from  the  abdominal  viscera  by  a  muscu- 
lar membrane,  the  diaphragm,  which  thus  divides  the  body 
cavity  into   thoracic  and  abdominal   cavities.     There  is    no 
cloaca,  the  vent  and  the  openings  of  the  urino-genital  systems 
are  separate.     The  eggs  are  fertilized  in  the  oviduct  and  develop- 
ment continues  for  a  variable  period,  up  to  two  years  in  the 
elephant,  in  an  enlargement  of  the  oviduct  called  the  uterus. 
The  embryo  is  provided  with  the  fcetal  membranes,   amnion 
and  allantois.     The  sense  organs  are  usually  highly  developed 
and  the  brain,  especially  the  cerebral  and  cerebellar  parts,  is 
much  in  advance  of  those  of  all  other  animals. 

652.  The  thirteen  orders  belong  to  three  sub-classes  as  follows: 
Sub-class  Monotremata.     Spiny  ant-eater,  duck  mole. 


302 


CLASSIFICATION   OF   ANIMALS 


Sub-class  Marsupialia. 

Order  i .  Polyprotodontia  (carnivorous  or  omnivorous) . 

Order  2.  Diprotodontia  (herbivorous).     Kangaroo. 
Sub-class  Monodelphia. 

Order  i.  Insectivora.     Moles,  shrews,  hedgehog. 

Order  2.  Chiroptera.     Bats. 


Opossum. 


FIG.  176. — The  spiny  ant-eater,  Tachyglossus.     Xi/4- 


FIG.  177. — The  duck-bill,  Ornithorhynchus.     Xi/4. 

Order  3.  Rodentia.     Rats,  mice,  rabbits,  squirrels,  beavers. 
Order  4.  Edentata  Nomarthra.     Scaly  ant-eater. 
Order  5.  Edentata  Xenarthra.     Sloth,  armadillo. 
Order  6.  Carnivora,     Dogs,  bears,  cats,  seals. 
Order  7.  Cetacea.     Whales,  porpoises,  dolphins. 


MAMMALIA  303 

Order  8.  Ungulata.     Hoofed  animals. 
Order  9.  Sirenia.     Manatee  and  dugong. 
Order  10.  Primates.     Monkeys,  apes,  man. 

653.  The  Monotremata  are  the  most  primitive  mammals.     The  embryo 
is  not  nourished  in  a  uterus.     An  egg  is  laid  from  which  the  embryo  hatches 
in  a  very  immature  condition.     It  is  then  nourished  from  the  mammary 
glands.     The  long  snout  is  covered  by  a  horny  sheath  like  a  duck's  bill. 
There  are  no  teeth  in  the  adult.     The  mammary  glands  have  no  nipple. 
Many  reptilian  characters  are  presented  as,  e.  g.,  in  the  presence  of  a 
coracoid  bone,  a  cloaca,  a  variable  body  temperature,  the  slightly  devel- 
oped  corpus   callosum,  the  condition  of  the  reproductive  organs,  etc. 
The   Monotremes   are   found    only   in   Tasmania,    Australia   and   New 
Guinea. 

654.  The  Marsupialia,  with  exception  of  the  American  opossum,  are 
also  confined  to  Australasia.     There  is  no  placenta  and  the  young  are 
born  in  a  very  immature  stage.     They  are  then  placed  in  a  sack  formed 
by  a  fold  of  the  skin  covering  the  region  of  the  mammary  glands.     Here 
the  young  attach  themselves  to  a  nipple  and  continue  their  development. 
The  pouch  is  supported  by  two  bones. 

655.  The  Polyprotodontia  are  carnivorous  or  omnivorous  Marsupials. 
They  have  a  well-developed  set  of  teeth  of  the  four  kinds. 

656.  The  Diprotodontia  are  vegetable  feeders  and  the  teeth  are  not 
developed  as  a  full  set. 

657.  The  Monodelphia  have  no  pouch.     The  fcetal  membranes  form  a 
placenta  which  becomes  attached  to  the  wall  of  the  uterus  thus  forming 
an  organic  connection  between  the  embryo  and  the  tissues  of  the  mother. 
By  means  of  the  placenta  the  embryo  is  nourished  for  a  period  within  the 
uterus.     After  birth  it  is  nourished  from  the  milk  of  the  mammary  glands. 

658.  The  Insectivora  have  a  full  set  of  pointed  teeth.     The  feet  usually 
are  five  toed  and  the  toes  clawed.     The  foot  is  plantigrade. 

659.  The  Chiroptera  have  wings  formed  by  a  membrane  of  skin  stretch- 
ing between  the  greatly  elongated  fingers  and  the  side  of  the  body.     In 
some  cases  the  eyes  are  large  and  the  ears  small,  in  others  the  eyes  are 
small  and  the  ears  large.     Some  are  fruit  eaters,  others  catch  insects.     A 
few  are  blood  sucking. 

660.  The  Rodentia  are  characterized  by  the  long,  sharp,  chisel-shaped 
incisors.     There   are    no    canines.     Rabbits,    squirrels,    beaver,    pocket 
gopher,  mice,  rats,  guinea  pig. 

66 1.  The  Edentata  are  either  without  teeth  or  have  poorly  developed 
teeth  without  enamel.     Scaly  ant-eater,  armadillo,  ant  bear. 


304  CLASSIFICATION   OF   ANIMALS 

662.  The  Carnivora  are  flesh-eating  mammals  with  a  characteristic 
dentition.     The  clavicle  is  rudimentary  or  wanting.     The  toes  are  clawed. 

663.  Sub-order   Fissipedia. — Terrestrial  carnivores  with   molar   teeth 
unlike.     Canidae;    dogs,    wolves    (digitigrade).     Ursidae;  bears,   (planti- 
grade).    Procyonidae;  raccoon,  (plantigrade).     Mustelidae;  weasel,  ferret, 
mink,  pole  cat.     Viverridae;  civet  cat  (ichneumon).     Hyasnidae;  hyaena. 
Felidae;  cats,  lions,  tigers,  leopards,  panther. 

664.  Sub-order  Pinnipedia. — Aquatic  carnivores  with  webbed  feet  and 
molar  teeth  alike.     Anterior  and  posterior  appendages  well  developed. 
Otters,  walrus,  seals. 

665.  The  Cetacea  are  aquatic  mammals  with  anterior  appendages  in 
form  of  paddles,  the  posterior  ones  only  represented  by  internal  rudiments. 
There  is  a  broad  horizontal  tail  fin.     In  the  baleen  whale  the  teeth  are 
wanting;  a  curtain  of  fringed  horny  plates  suspended  from  the  roof  of  the 
mouth  acts  as  a  strainer  to  collect  the  food.     Whales,  narwhal,  dolphins. 

666.  The  Ungulata  have  broad  toes  covered  with  a  horny  hoof.     Sub- 
order Proboscidia;  elephants,  5  toes,  thick  skin,  long  proboscis.     Sub- 
order Perissodactyla ;  number  of  toes  odd.     Tapir,  4  toes  on  anterior 
appendages,  3  on  the  posterior.     Rhinoceros,  3  toes.     Horse,  i  toe.     Sub- 
order Artiodactyla,  even  number  of   toes.     Section  i.  Non-ruminants: 
Hippopotamus,  4  toes;  swine,  2  long,  2  short  toes.     Section  2.  Ruminants: 
Tribe  i.  Tylopoda;  camel,  dromedary,  llama,  toes  2,  no  horns  or  antlers. 
Tribe  2.  Traguloidea;    toes  2  long,   2  short    (small,  hornless,  deer-like 
animals  of  West  Africa).     Tribe  3.  Pecora;  usually  with  horns  or  antlers, 
toes  2  long,  2  rudimentary.     Deer  family  with  antlers.     Cattle  family 
with  horns;  antilope,  buffalo,  cattle,  goats,  sheep.     Giraffe  family  with 
two  toes,  horns  covered  with  skin. 

Order  Ungulata: 

Sub-order  i.  Condylarthea.     Phenacodus,  extinct. 
Sub-order  2.  Hyracoidea.     Hyrax — the  coneys  of  Africa  and  Ara- 
bia. 

Sub-order  3.  Proboscidea.  Elephants. 

Sub-order  4.  Perissodactyla  (odd-toed).    Tapirs,  rhinoceros,  horse. 
Sub-order  5.  Artiodactyla  (even-toed). 

Section  i.  Non-ruminantia.     Hippopotamus,  swine. 
Section  2.  Ruminantia: 
Tribe  i.  Tylopoda. 
Tribe  2.  Traguloidea. 
Tribe  3.  Pecora. 


MAMMALIA  305 

667.  The   Sirenia   are   herbivorous   marine   mammals.     The    anterior 
appendages  are  paddle  like,  the  posterior  rudimentary.     Manatee,  dugong, 
sea  cow. 

668.  The  Primates  have  a  heterodont  dentition,  all  appendages  have 
five  digits.     The  nails  are  flat.     The  eyes  are  directed  forward.     Sub- 
order Prosimiae;  the  teeth  similar  to  those  of  Insectivores,  the  wall  of  the 
orbit  incomplete  laterally.     Lemurs.     Sub-order  Simiae;  the  incisors  are 
chisel  shaped,  the  orbit  wall  complete.     Monkeys  and  apes. 

Section  i.  Platyrhina;  the  flat  nose  monkeys  of  South  America. 
Section  2.  Catarrhina;  the  monkeys,  mandrels,  gibbons,  orang- 
utan, chimpanzie  and  gorilla. 

669.  Finally  the  genus  Homo  is  placed  by  some  authors  in  an  order  by 
itself,  the  order  Bimana.     Others  place  this  genus  in  a  family,  Hominidae, 
under  the  order  Primates.     The  genus  is  regarded  as  containing  only  one 
living  species,  Homo  sapiens,  which  is  sub-divided  into  races. 


PART  III.-GENERAL  PRINCIPLES 

THE  CELL  AND  THE  INDIVIDUAL 

670.  Spontaneous  Generation. — It  was  at  one  time  held  that 
some  animals  originate  spontaneously.     In  the  middle  of  the 
seventeenth  century  the  great  anatomist,  Harvey,  expressed 
the  view  that  all  living  things  spring  from  eggs  (Omne  vivum 
ex  ovo).     But  this  opinion  was  not  generally  accepted.     A 
quarter  of  a  century  later  another  anatomist,  Redi,  showed  that 
the  maggots  which  develop  in  decaying  flesh  are  bred  from  the 
eggs   deposited  by  flies.     But  for  two  centuries  more  spon- 
taneous generation  was  thought  to  account  for  the  appearance 
of  many  living  things,  though  it  came  gradually  to  be  limited 
to  the  microscopic  organisms,  like  the  bacteria  and  protozoa. 
Finally,  in  the  latter  half  of  the  nineteenth  century  the  experi- 
ments of  Pasteur  and  others  definitely  established  the  view  that 
even  for  these  microscopic  forms  a  living  germ  is  necessary  to 
development  of  a  living  organism.     It  was  shown  that  if  the 
germs  of  the  organisms  which  produce  fermentation  and  decay 
were  carefully  excluded  from  the  substances  in  which  they 
usually  occur,  that  the  processes  of  fermentation  and  decay 
would  not  take  place  and  the  associated  organisms  would  not 
appear. 

671.  Continuity  of  the  Living  Substance. — At  the  present 
time  the  term  egg  is  applied  only  to  special  cells  produced  by 
multicellular  organisms,  from  which  new  individuals  develop, 
but   the   unicellular  organisms  produce  spores  which  are,  in 
this   sense,    the   counterpart   of  eggs.     Another  objection  to 
Harvey's  phrase  may  be  made  on  the  ground  that  new  individ- 
uals may  be  produced  by  methods,  such  as  budding,  fission, 

307 


308  GENERAL  PRINCIPLES 

etc.,  in  which  neither  eggs  nor  spores  occur.  Still,  in  essence, 
it  is  now  generally  accepted  as  true  that  all  living  things 
originate  from  eggs,  and  in  this  statement  is  expressed  one  of 
the  most  remarkable  attributes  of  the  living  substance,  that  of 
its  continuity. 


FIG.  178.  Amoeba  vespertilio,  showing  the  structure  of  the  protoplasm.  The 
outer  layer  is  denser  and  more  homogeneous,  and  is  called  ectoplasm.  The 
central  part,  called  endoplasm,  has  the  appearance  of  foam.  (From  Marshall 
after  Doflein.) 


672.  Structure  of  Protoplasm. — Much  has  been  learned  con- 
cerning the  physical  and  chemical  properties  of  protoplasm, 
but  even  our  best  microscopes  and  most  refined  chemical 
methods  still  leave  much  more  to  be  determined.  As  seen 
through  the  microscope  the  cytoplasm  seems  to  consist  of:  (i) 
a  ground  substance  of  a  transparent,  colorless,  homogeneous 


PROTOPLASM 


309 


fluid,  and  (2)  a  network  of  a  more  viscid  and  more  highly 
refractive  and  sometimes  granular  substance.  This  network 
may  be  either  the  sectional  view  of  a  sponge-like  structure  or 
of  an  emulsiform  structure  or  of  true  fibres  interlaced.  The 
granules,  which  are  called  microsomes,  vary  greatly  in  size  and 
number  and  are  frequently  absent  or  too  small  to  be  seen.  They 
are  often  less  conspicuous  in  the  peripheral  layers  of  the  cyto- 
plasm, which  is  therefore  distinguished  as  ectoplasm. 

(Attraction-sphere  enclosing  two  centrosomes.) 


Plastids  lying 
in  the  cyto- 
plasm 


Vacuole 


Passive  bodies 
(metaplasmor 
paraplasm) 
suspended  in 
the  cyto- 
plasmic  mesh- 
work 


FIG.  179. — Diagram  of  a  cell.     (From  Hegner's  Zoology,  after  Wilson,  published 
by  the  Macmillan  Co.) 

Beside  these  constant  constituent  elements  of  the  cytoplasm 
there  are  also  a  large  number  of  structures,  which  occur  only 
in  certain  cells  or  at  certain  times.  Among  these  may  be  men- 
tioned here  the  chromoplasts  and  amyloplasts,  the  vacuoles 
filled  with  cell  sap,  the  ingested  food  particles,  the  reserve  elabor- 
ated food  substances,  such  as  starch,  oil,  aleurone,  and  the  se- 


310  GENERAL   PRINCIPLES 

cretions  and  other  like  substances  resulting  from  the  activity  of 
the  protoplasm.  With  the  last  named  class  of  cell  constituents 
may  be  included  the  cell  membrane  or  cell  wall.  This  is  usually 
present  in 'plants  and  usually  absent  in  animal  tissue,  though 
many  exceptions  occur  to  both  rules. 

673.  The  Nucleus. — The  nucleus  must  be  regarded  as  an 
essential  part  of  the  cell.     It  is  true  there  are  certain  lowly 
organisms,   such  as  the  bacteria,  blue  green  algae  and  related 
forms,   in  which  there  is  no   such  distinct,   highly    complex 
structure  as  the  typical  nucleus;  but  even  in  these  forms  there 
are  found  scattered  in  the  protoplasm  of  the  cell  minute  bodies 
which  have  properties  recognized  as  belonging  to  constituents 
of  the  nucleus.     These  are  generally  regarded  as  representing 
the  nucleus. 

674.  The  typical  nucleus  is  a  round  or  oval  body,  but  it  may 
also  be  greatly  elongated  or  even  branched.     It  is  usually  single, 
but  sometimes  it  consists  of  two  parts,  a  large  macronucleus 
and  a  small  micronucleus.     Sometimes  there  are  several  or 
even  many  nuclei  in  one  cell.     Usually  the  nucleus  is  provided 
with  a  membrane,  but  this  disappears  at  certain  times,  and  in 
some  cases  is  entirely  absent.    Like  the  rest  of  the  protoplasm, 
the  nucleus  is  transparent  and  colorless,  and  in  the  living  condi- 
tion appears  homogeneous.     But  if  the  cell  is  treated  with  cer- 
tain  "fixing"    and   staining   reagents,    the   nucleus   becomes 
deeply  colored,  due  to  the  affinity  of  one  of  its  constituents  for 
the  dye.     Because  of  its  tendency  to  stain,  this  substance  is 
called  chromatin.     In  the  "resting"  nucleus   the   chromatin 
assumes  a  great  variety  of  forms;  sometimes  it  is  in  the  form  of 
granules  of  various  sizes,  more  often  it  is  best  described  as  a 
mass  of  knotted  and  tangled  threads.     The  chromatin  is  ap- 
parently a  very  important  constituent  of  the  cell,    and   in   it 
centre  many  most  interesting  phenomena.     Another  element 
of  the  nucleus  which  may  be  stained  is  the  nucleolus.     There 
is  often  only  one,   but  there  may  be  more.     They  are  usu- 


THE    CELL  311 

ally  spherical  masses  and,  therefore,  distinguishable  from  the 
chromatin.  But  a  better  means  of  distinguishing  between 
these  is  given  by  the  fact  that  they  are  not  stained  by  the 
same  dyes. 

675.  The  linin  is  a  part  of  the  nucleus  which  does  not  stain 
at  all  by  ordinary  methods.     It  also  assumes  various  forms, 
but  when  most  evident  it  is  as  a  system  of  fibres,  or  a  network 
of  threads  by  which  the  other  elements  of  the  nucleus   are 
bound  together. 

676.  The  interstices  of  the  nucleus  are  filled  with  a  nuclear 
sap. 

677.  There  is  one  other  structure  in  the  cell  which  must  be 
mentioned,  though  there  is  some  doubt  whether  it  should  be 
classed  with  the  cytoplasmic  or  nucleoplasmic  structures.     This 
is  the  centrosome.     It  is  generally  found  in  the  cytoplasm  close 
by  the  side  of  the  nucleus,  but  sometimes  it  is  far  removed,  and 
again  it  seems  to  be  enclosed  by  the  nuclear  membrane.     The 
centrosome  is  excessively  small,  scarcely  more  than  a  point, 
even  with  the  highest  powers  of  the  microscope,  but  it  may  be 
stained  by  certain  methods,  and  is  further  distinguished  from 
other  minute  protoplasmic  structures  by  the  radial  arrange- 
ment of  the  surrounding  protoplasm,  for  which  it   forms   a 
centre.     Something  concerning  its  significance  will  appear  in 
the  discussion  of  cell  division. 

678.  In  this  brief  description  of  the  protoplasm,  only  the 
most  important    constant  structures    have  been   mentioned. 
These  have  each  their  optical,  physical,  and  chemical  peculi- 
arities.    The  list  of  substances  which  have  been  recognized 
might  be  greatly  extended  and  yet,  because  of  our  imperfect 
instruments  and  methods  we  are  far  from  having  made  a  com- 
plete analysis  of  the  protoplasm.     It  seems  probable  that  fur- 
ther investigation  will  show  that  among  the  innumerable  mi- 
nute particles  which  at  present  are  indistinguishable  one  from 
another  are  many  chemically  and  otherwise  distinguishable  kinds. 


312  GENERAL   PRINCIPLES 

679.  Chemical  Structure  of  Protoplasm. — Protoplasm  con- 
tains a  very  large  percentage  of  water,  70  per  cent  or  more;  it 
is  alkaline  in  reaction  in  the  living  condition  and  contains  many 
mineral  salts,  which,  however,  vary  greatly  with  the  kind  of 
protoplasm.     Among   the   chemical  elements   which   may  be 
found    are    phosphorous,    manganese,    magnesium,    calcium, 
sodium,  chlorine,  and  iron.     It  does   not   necessarily   follow 
that  these  substances  form  an  integral  part  of  the  protoplasmic 
molecule.     They  may  be  present  as  inorganic  salt  dissolved  in 
the  cell  sap  or  even  in  crystalline  form.     Chemically,  the  living 
substance  is  classed  with  the  albumens,  but  it  were  perhaps 
better  to  say  that  on  analysis  it  decomposes  into  a  series  of  albu- 
minous compounds.     These  are  themselves  extremely  complex 
organic  bodies,  and  as  yet  lie  somewhat  beyond  the  range  of  the 
chemist's   power   of   analysis.     An   analysis   of   egg   albumen 
yielded  the  result,  C72Hio6Ni8SO22,  though  this  cannot  be  re- 
garded as  a  correct  chemical  formula.     Nucleoplasm  is  dis- 
tinguished from  the  cytoplasm  by  the  presence  of  phosphorous. 
From  all  that  we  know  regarding  protoplasm  we  are  led  to  re- 
gard it  as  an   aggregate    of    many   highly   complex    organic 
compounds. 

680.  Function  of  Cytoplasm  and  Nucleus. — Much  light  has 
been  thrown  on  the  question  of  the  function  of  cytoplasm  and 
nucleus  by  a  series  of  simple  experiments.     If  a  unicellular 
organism  is  cut  into  two  parts,  so  that  the  nucleus  is  also 
divided,    the   wound    immediately    " heals,"    and    each    half 
regenerates  the  part  cut  away  so  that  eventually  there  are  two 
complete  organisms.     The  operation  does  not  itself  greatly 
injure  the  cell.     If  the  cell  is  divided  so  that  all  of  the  nucleus 
remains  in  one  part,  the  part  without  nuclear  matter,  even 
though  it  is  the  larger  part,  does   not   regenerate.     It   may 
remain  alive  for  weeks,  but  ultimately  dies.     If  the  nucleus  is 
completely  removed  from  the   cytoplasm,  both  nucleus  and 
cytoplasmic  parts  die.     The  death  of  enucleated  portions  is 


THE    CELL  313 

evidently  due  to  the  lack  of  nutrition.  The  cytoplasm  retains 
its  irritability  and  responds  to  stimuli;  the  pseudopodia  may 
still  be  formed  or  the  cilia  continues  to  move,  as  the  case 
may  be,  but  food  particles  are  no  longer  ingested,  and  those 
contained  in  the  food  vacuoles  are  no  longer  digested.  The 
cytoplasm  seems  to  have  lost  the  function  of  assimilation  and 
consequently  starves. 

68 1.  These  experiments  indicate  that  the  animal  functions, 
irritability  and  contractility,  are  functions  of  the  cytoplasm. 

682.  On  the  other  hand,  from  what  has  just  been  said,  it  is 
seen  that  the  nucleus  has  to  do  with  the  function  of  assimilation. 
This  is  further  evident  in  many  cases  in  which  cells  are  especially 
active  in  the  absorption  of  food.     In  such  cases  the  nucleus  is 
prolonged  into  curious  finger-like  processes  on  the  side  of  special 
activity.     In  other  cases  the  nucleus  shows  evidence  of  special 
activity  where  secretory  processes  are  prominent.     Here  its 
surface  also  projects  toward  the  point  of  activity.     Since  assimi- 
lation and  secretion  are  two  phases  of  metabolism  it  is  natural 
that  both  should  be  controlled  from  the  same  source,  and  that 
the  nucleus  should  present  similar  appearances  in  both  cases. 
In  addition  to  the  control  of  metabolism  the  nucleus  also  has 
the  function  of  cell  division  or  reproduction.. 

683.  Cell  Division. — The  process  of  cell  division  is  such  a 
complicated  one,  and  with  it  are  connected  so  many  important 
biological  phenomena  that  it  demands  careful  study.     The  first 
evidence  of  preparation  for  cell  division  is  seen  in  the  rearrange- 
ment of  the  chromatin.     This  gradually  assumes  a  more  regular 
form.     The  irregular  clumps  and  strands  take  on  the  form  of 
one  or  more  coiled  bands.     These  have  at  first  an  irregular  out- 
line, which  gradually  becomes  smoother.     The  bands  become 
thicker  and  shorter  and  finally  are  seen  to  consist  of  a  limited 
and  definite  number  of  V-shaped  bodies,  to  which  the  name 
chromosomes  has  been  given.     During  these  changes  of  form 
the  affinity  of  the  chromatin  for  the  stains  increases.     At  about 


3*4 


GENERAL   PRINCIPLES 


this  time  two  centrosomes  make  their  appearance  close  beside 
the  nucleus.  They  are  connected  by  a  number  of  slender  fibres, 
which  curve  outward  in  the  middle  so  that  all  together  form 
a  spindle-shaped  figure.  The  centrosomes  gradually  recede 


FIG.  180. — Diagrams  illustrating  the  prophases  of  mitosis.  A,  Beginning  of 
formation  of  the  spindle.  B,  chromosomes  formed.  C,  Chromosomes  approach- 
ing the  equator  of  the  spindle.  D,  Chromosomes  ready  to  divide.  (See  next 
figure.) 

from  each  other  until  they  come  to  lie  at  opposite  poles  of  the 
nucleus.  During  these  changes  the  nucleoli  have  gradually  dis- 
appeared and  the  nuclear  membrane  also  becomes  indistinct 
and  fades  away.  The  spindle  fibres  now  traverse  the  nucleus 


CELL   DIVISION 


315 


and  some  are  attached  to  the  chromosomes.  Other  fibres  ex- 
tend from  the  centrosomes  outward  into  the  cytoplasm,  and 
the  constituents  of  the  cytoplasm  take  on  a  radial  arrangement 
with  the  centrosomes  as  centres.  The  chromosomes  lie  regu- 
larly arranged  around  the  spindle  in  its  equatorial  plane. 


FIG.  181. — Diagrams  illustrating  the  metaphase  and  anaphases  of  mitosis. 
A,  Chromosomes  divided;  B,  chromosomes  approaching  the  centrosomes;  C, 
cytoplasm  beginning  to  divide,  the  centrosomes  also  divided;  D,  cell  completely 
divided  and  nuclei  in  resting  condition.  (This  and  preceding  figure  from 
McMurrich,  adapted  from  E.  B.  Wilson.) 

684.  The  process  of  division  begins  with  the  splitting  of  each 
chromosome  lengthwise  into  two  equal  and  similar  parts, 
whereby  the  number  of  chromosomes  is  doubled.  The  two 
halves  of  each  original  chromosome  separate  and  move  in 


316  GENERAL   PRINCIPLES 

opposite  directions,  each  one  approaching  one  of  the  centro- 
somes.  In  this  way  two  groups  of  chromosomes  of  equal 
numbers  are  formed  at  opposite  ends  of  the  spindle.  At  about 
this  time  a  groove  appears  in  the  surface  of  the  cytoplasm  in 
the  equatorial  plane  of  the  spindle.  This  groove  cuts  deeper 
into  the  cell  until  it  is  divided  into  two  equal  masses.  Thus  the 
nucleus  and  cytoplasm  are  divided. 

685.  The  process  of  division  is  concluded  by  the  formation  of 
a  nuclear  membrane  around  each  group  of  chromosomes  and  the 
rearrangement  of  the  chromatin.     The  chromosomes  lose  their 
individuality  again  in  a  tangle  of  chromatin  and  the  nucleoli 
reappear.     The  spindle  fibres  and  attraction  sphere  disappear 
and  the  centrosomes  may  also  be  lost  among  the  other  granules 
of  the  cytoplasm. 

686.  This  process  of  cell  division  is  called  mitosis  or  karyo- 
kinesis.     It  is  the  normal  method  of  cell  division,  but  under 
certain  conditions  a  simpler  process  occurs.     This  has  been 
described  elsewhere.     With  slight  modification  the  description 
just  given  of  the  mitotic  method  will  apply  generally  to  both 
animals    and    plants.     Several    additional    points    may    be 
mentioned. 

687.  Number  of  Chormosomes. — The  number  of  chromo- 
somes is  constant  for  any  given  species.     For  different  species 
the  numbers  observed  vary  from  two  in  the  Nematode,  Ascaris 
megalocephala,  to  168  in  Artemia,  a  genus  of  Crustacea.     The 
most  common  numbers  recorded  are  12,  16  and  24. 

688.  Nucleoli. — The  fate  of  the  nucleoli  during  mitosis  is 
in  question.     There  is  some  reason  for  believing  that  in  some 
cases  at  least  they  take  part  in  the  formation  of  the  chromo- 
somes.     More  often  they  seem  to  disintegrate,  and  then  new 
ones  are  formed  on  the  organization  of  the  new  nucleus. 

689.  Centrosomes. — The  centrosomes  are  apparently  per- 
manent cell  structures  which  propagate  themselves  by  divi- 
sion.    At  the  close  of  cell  division  the  centrosome  of  e< 


CELL   DIVISION  317 

daughter  cell  often  divides  before  it  is  lost  in  the  granular  pro- 
toplasm of  the  resting  cell.  In  many  cases  the  centrosome  or 
centrosomes  can  be  found  in  the  resting  cells.  In  other  cases, 
division  of  the  centrosome  occurs  just  previous  to  cell  division. 

690.  Spindle  Fibres. — The  spindle  fibres  are  achromatic  sub- 
stances, i.  e.,  they  do  not  stain  like  the  chroma  tin.     They  are 
probably  derived  in  large  part  from  the  linin  network  of  the 
nucleus.     The  astral  figures  and,  perhaps,  the  spindle  in  part, 
are  cytoplasmic. 

691.  Resting  Nucleus. — Ordinarily,  after  division,  the  cells 
remain  quiescent  for  a  time;   that  is,  so  far  as  any  apparent 
changes  in  the  nucleus   are   concerned.     This  is    called   the 
resting  state,  although  growth  and   other  processes  may  be 
actively  going  OP. 

692.  Conjugation. — Another    very    important    process    in 
which  the  nucleus  is  especially  concerned  is  the  reverse  of  cell 
division.     Cell  division  is  ordinarily  followed  by  a  period  of 
growth,  then  another  cell  division  and  another  period  of  growth. 
This    constitutes    the  ordinary    daily    routine  of  cell  life.     At 
comparatively  long  intervals  this  chain  of  events  is  broken  by  the 
fusion  of  one  cell  with  another.     This  may  take  place  in  a  great 
variety  of  ways,  which,  however,  are  apparently  fundamentally 
the  same,  and  the  significance  of  the   process  may  best  be 
explained  by  the  description  of  a  number  of  examples. 

693.  Mougeota  is  a   filamentous  alga,  which  consists  of  a 
series  of  similar  cells  adhering  to  each  other  in  a  row.     At 
certain  times  two  such  filaments,  lying  side  by  side,  become  con- 
nected by  a  series  of  tubes  in  such  a  way  that  each  cell  of  one 
filament  is  connected  with  a  similar  cell  in  the  other  filament. 
Then  the  protoplasm  of  each  cell  flows  into  the  tube  where  the 
two  masses  unite  into  a  single  body.     In  some  other  algae  swarm 
spores     are     formed    and     set    free    in    the     water.     Each 
spore  is  pear-shaped  and  has  two  flagella  by  which  it  actively 
swims  about.     They  are  all  of  the  same  size  and  cannot  be 


GENERAL    PRINCIPLES 


distinguished  one  from  the  other.  These  swarm  spores  unite  in 
pairs,  and  from  each  pair  a  single  protoplasmic  mass  is  formed, 
which  is,  in  fact,  a  single  cell.  Spirogyra  is  another  filamentous 
alga  similar  to  Mougeota,  but  when  two  filaments  unite  by 
tubes  the  protoplasts  of  one  flow  completely  across  to  the  other, 
where  the  union  takes  place;  the  protoplasts  of  the  second  cell 
remaining  passive.  In  another  example  of  swarm  spores,  two 
kinds  of  spores  are  produced.  The  difference  is  not  great,  but 


FIG.  182. — Conjugation  and  differentiation  of  sex.  A,  Conjugation  in  Mou- 
geota; B,  conjugation  in  Spirogyra;  C,  conjugation  in  Hydrodictyon  (isogamous) ; 
D,  conjugation  in  Chlamydomonas  (heterogamous).  In  C:  i,  a  gamete;  2,  con- 
jugation; 3,  zygote.  In  D:  i,  a  male  gamete;  2,  female  gamete. 

one  is  somewhat  larger  than  the  other.  Here  the  union  takes 
place  between  individuals  of  different  kind.  In  Fucus,  a 
brown  sea  weed,  the  two  kinds  of  cells  differ  greatly  in  size. 
The  large  one  is  motionless,  while  the  small  one  has  two 
flagella  and  is  very  active.  In  all  the  higher  plants  and 
animals  the  difference  in  size  is  enormous,  and  in  animals  espe- 
cially, the  smaller  motile  cell  consists  of  little  more  than 
a  small  and  compact  nucleus  with  a  single  flagellum. 

694.  Fertilization. — In  the  lower  forms,  where  the  two  unit- 
ing cells  are  equal  in  size  or  nearly  so,  the  process  in  question  is 


FERTILIZATION  319 

called  conjugation;  in  those  instances  where  there  is  a  great 
difference  in  size,  it  is  called  fertilization.  The  conjugating 
elements  are  called  gametes.  In  case  of  fertilization  the  large 
cell  is  called  a  macrogamete  or  egg,  while  the  small  one  is  called 
a  microgamete  or  sperm.  Further,  the  individual  giving  origin 
to  a  macrogamete  is  called  female,  and  the  one  from  which  the 
microgamete  springs  is  a  male.  A  further  comparison  between 
male  and  female  will  be  made  a  little  later. 

695.  Maturation. — In  every  case  of  conjugation  or  fertili- 
zation the  nuclei  of  the  two  cells  sooner  or  later  unite,  and  the 
real  significance  of  the  process  centres  in  the  nuclei.  The  state- 
ment has  already  been  made  that  the  number  of  chromosomes 
is  fixed  for  a  given  species,  a  condition  that  is  maintained  by  the 
splitting  of  the  chromosomes  at  each  cell  division.  On  the 
fusion  of  the  nuclei,  however,  the  number  would  be  doubled 
were  it  not  for  the  preliminary  process  by  which  both  nuclei 
are  prepared  for  the  approaching'fusion.  To  explain  this  proc- 
ess we  will  take  as  an  example  the  sperm  cells  of  Ascaris  megalo- 
cephala.  The  typical  number  of  chromosomes  in  this  species 
is  four,  and  this  is  also  the  number  in  the  cell  divisions  which 
lead  up  to  the  formation  of  the  sperm  mother  cells.  The  latter 
remain  for  an  unusually  long  period  in  the  growing,  resting 
condition  and  attain  unusually  large  size.  When  mitosis  begins 
it  is  seen  that  the  four  chromosomes  have  already  split  into 
eight  which,  however,  still  remain  paired.  The  four  pairs  ar- 
arnge  themselves  in  two  groups  of  four  (tetrads)  in  the  equator  of 
the  spindle,  and  the  cell  divides  into  two  halves,  after  the  usual 
method.  Then  the  centrosomes  immediately  divide  and  form 
new  spindles,  so  that  a  second  division  occurs  before  the  nucleus 
has  entered  the  resting  condition.  In  this  second  division  the 
four  daughter  chromosomes  do  not  split,  but  one  chromosome 
from  each  of  the  original  two  tetrads  moves  toward  each  pole  of 
the  spindle.  There  are  thus  formed  four  similar  cells,  each 
with  two  chromosomes.  These  are  the  sperm  cells. 


320 


GENERAL   PRINCIPLES 


696.  Ill  the  development  of  the  egg  the  process  is  similar  to 
that  of  the  sperm  up  to  the  period  of  growth.  The  mother  cell 
of  the  egg  continues  to  grow  for  a  long  time,  so  that  a  cell  of 
comparatively  gigantic  proportions  results.  When  the  nucleus 
proceeds  to  divide,  four  pairs  of  chromosomes  appear,  as  in  the 


FIG.  183. — Diagram  illustrating  the  reduction  of  the  chromosomes  during 
spermatogenesis.  (McMurrich.)  scl,  Spermatocyte  of  the  first  order;  sc2, 
spermatocyte  of  the  second  order;  sp,  spermatid.  The  number  of  the  chromo- 
somes is  supposed  to  be  8(  =  2x)  in  the  zygote,  and  4(  =  x)  in  the  gametes. 

sperm  mother  cell,  and  they  are  likewise  arranged  in  two  groups 
of  four.  The  nucleus,  however,  comes  to  the  surface  of  the  cell 
and  the  spindle  takes  a  radial  position,  so  that  on  division  of 
the  cell  there  results  one  very  small  cell  (first  polar  body)  and 
one  very  large  one.  Then,  as  in  the  case  of  the  sperm,  the 


MATURATION 


321 


nucleus  of  the  large  cell  divides  again  without  an  intervening 
resting  stage,  and  a  second  polar  body  is  formed.  As  a  result 
of  these  two  divisions  there  are  now  one  large  cell — the  ripe 


FIG.  184. — Diagram  illustrating  the  reduction  of  the  chromosomes  during  the 
maturation  of  the  ovum.  (McMurrich.)  o,  Ovum;  ocl,  oocyte  of  the  first 
generation;  oc2,  oocyte  of  the  second  generation;  p,  polar  globule.  The  number 
of -the  chromosomes  is  supposed  to  be  8(  =  2x)  in  the  zygote,  and  4(  =  x)  in  the 
gametes. 

egg — and  two  small  ones.  The  nucleus  of  the  egg  cell  has  two 
chromosomes,  and  so  has  the  second  polar  body.  If  the  first 
polar  body  were  to  divide  in  the  same  way,  there  would  have 
been  three  polar  bodies,  each  with  two  chromosomes.  This 

21 


322  GENERAL  PRINCIPLES 

actually  takes  place  in  many  animals,  but  the  polar  bodies 
undergo  no  further  development  and  are  to  be  regarded  as 
parts  ejected  from  the  maturing  egg  cell  as  useless.  It  is  also 
reasonable  to  regard  the  first  polar  body  of  Ascaris  as  poten- 
tially equivalent  to  two  second  polar  bodies.  This  leads  then 
to  the  conclusion  that  the  egg  nucleus  and  the  nuclei  of  the 
polar  bodies  are  the  equivalents  of  the  four  sperm  nuclei  which 
developed  from  one  sperm  mother  cell. 

697.  The  essential  difference  between  the  ripe  egg  cell  and 
the  sperm  lies  in  the  great  size  of  the  egg  and  the  motility  of  the 

Spermato 
Oocyte  I  (       )  (       )  cyte  I 


Oocytell 


Ovum 


O  O        O  O     O  O     Ospermatids 


Polar  globules 


FIG.  185. — Diagram  to  show  the  similarity  in  the  development  of  ova  and  sperm 
cells.     (McMurrich.) 

sperm.  The  great  size  of  the  egg  is  due  in  part  to  the  prolonged 
growth  period,  during  which  the  quantity  of  protoplasm  is 
greatly  increased  and  reserve  food  in  the  form  of  yolk  granules 
is  stored  up  within  the  cell,  and  in  part  to  the  formation  of  two 
(three)  rudimentary  cells  (polar  bodies)  in  the  process  of  matura- 
tion instead  of  four  equals  cell,  which  leaves  one  with  the  devel- 
opmental material  which  would  otherwise  be  divided  among 
the  four.  These  are  evidently  provisions  for  the  early  develop- 
mental period  of  the  embryo. 

698.  The  consequences  of  these  preparatory  processes  are 
now  readily  seen.     The  second  maturation  division  is  called 


MATURATION  323 

the  reduction  division,  because  it  leaves  the  nucleus  with  half 
the  normal  number  of  chromosomes.  When  now  fertilization 
takes  place  the  sperm  nucleus  and  the  egg  pro-nucleus  fuse,  and 
there  results  therefrom  a  new  egg  nucleus  with  the  normal  num- 
ber of  chromosomes.  In  many  ways  minor  variations  occur, 
but  the  essential  features  seem  to  hold  throughout  the  animal 
and  vegetable  kingdoms.  Only  one  variation  will  be  described. 

699.  Conjugation  in  Potozoa. — In  many  protozoa  conjuga- 
tion occurs  in  a  peculiar  form.     The  cells  do  not  fuse  and  only 
the  nucleus  is  greatly  affected  by  the  process.     In  Paramcecium, 
for  example,  two  individuals  adhere  to  each  other  by  their  oral 
surfaces  and  remain  in  this  condition  for  some  time.     Finally, 
they  separate,  the  same  two  individuals,  at  least  in  external  ap- 
pearance.    However,  radical  changes  have  occurred  in  the  nuclei, 
which  are  briefly  as  follows:     The  macronucleus  disintegrates 
and  finally  disappears  without  apparently  having  taken  any 
part  in  the  process  of  conjugation.     The  micronucleus  forms  a 
•spindle  and  divides.     This  is  repeated  so  that  four  daughter 
nuclei  are  produced.     Three  of  these  also  disintegrate  while  the 
fourth  divides  again.     There  are  now  two  active  nuclei  in  each 
cell,  and  one  of  these  passes  out  of  one  cell  to  the  other  through 
the  cell  mouths,  which  are  placed  one  over  the  other.     These 
nuclei  are  regarded  as  the  equivalents  of  sperm  nucleus  and  egg 
pro-nucleus,  and  each  is  said  to  have  approximately  half  the 
usual  chromatic  substance.     From  the  fusion  of  the  two  nuclei, 
the  receptive  nucleus  and  the  wandering  nucleus,  a  new  nucleus 
is  produced,  from  which  the  new  macronucleus  and  the  new 
micronucleus  are  both  developed. 

700.  Fertilization. — The  approach  of  the  sperm  to  the  egg 
cells  is  not  a  matter  of  accident.     Observation  of  the  movement 
of  the  active  sperm  in  the  presence  of  a  ripe  egg  is  sufficient  to 
persuade  one  that  there  is  a  positive  stimulus  which  directs  them 
to  the  egg.     In  a  short  time  large  numbers  are  swarming  about 
the  egg  and  apparently  endeavoring  to  penetrate  its  surface. 


324 


GENERAL   PRINCIPLES 


1 


o 


p 


CO 
CO 


8 


0 


€ 


€ 


c> 
€) 


00 


0  0 

oo 


0 

o 


o 


FIG.  1 86. — Diagram  of  the  process  of  conjugation  in  Paramcecium  (on  the 
left),  and  of  maturation  and  fertilization  (on  the  right).  In  A  and  /  the  out- 
lines of  the  cells  are  represented  and  the  condition  of  the  nuclei  before  and  after 
the  changes  they  undergo.  In  each  case  white  and  black  are  used  to  distinguish 
between  the  nuclear  material  of  the  two  parent  cells.  In  B-H  the  cell  out- 
lines are  omitted. 

In  Paramoeceum  (on  the  left);  A  and  B,  temporary  union  of  the  two  cells;  C, 


FERTILIZATION  325 

By  experiment  it  can  be  shown  that  the  sperms  of  mosses  are 
attracted  by  solutions  of  cane  sugar  and  fern  sperms  are  at- 
tracted by  solutions  of  malic  acid.  These  or  similar  sub- 
stances are  probably  excreted  by  the  egg  and  serve  as  a  direct- 
ing stimulus  to  the  sperm. 

701.  Many  eggs  are  enclosed  in  a  gelatinous  envelope,  which 
is  readily  penetrated  by  the  sperm.     In  eggs  which  have  a  firmer 
covering,  or  "shell,"  there  are  one  or  more  pores  (micropyles) 
through  which  the  sperms  enter.     In  most  cases  only  one  sperm 
cell  enters  the  egg  under  normal  conditions.     This  control  is 
effected  by  the  response  of  the  egg  to  the  first  sperm.     For  no 
sooner  has  a  sperm  entered  the  egg  than  the  latter  develops  a 
membrane  which  excludes  all  other  sperms. 

702.  Cleavage. — After  fertilization  the  eggs  of  higher  plants 
and  animals  immediately  begin  segmenting  (early  cell  division). 
Among  the  lower  forms,  on  the  contrary,  a  long  "resting  stage" 
often  ensues  after  the  zygote  (conjugated  gametes)  or  fertilized 
egg  has  formed  a  heavy  membrane. 

703.  In  some  instances  the  chromosomes  of  the  two  gametes 
can  be  separately  followed  through  the  first  cell  division.     In 
which  case  it  is  seen  that  the  chromosomes  of  the  first  segmen- 
tation spindle  come  in  equal  numbers  from  the  two  gametes 
and  that  therefrom  the  daughter  cells  receive  an  equal  number 
of  chromosomes  from  each  parent.     This  is  an  important  point 
which  will  be  discussed  more  fully  at  another  place.     The  events 
which  follow  the  first  cell  division  vary  greatly  with  the  organ- 
first  division  of  the  micronucleus;  D,  second  division;  E  and  F,  three  daughter 
nuclei  disintegrate,  the  fourth  divides  into  a  migrating  male  element  and  a  pas- 
sive female  element;  F  and  G,  the  male  element  migrates  into  the  other  cell  and 
fuses  with  the  opposite  female  element;  H,  the  process  completed;  /,  the  cells 
separate.     In  maturation  and  fertilization  (on  the  right) :  A  and  B,  the  primary 
spermatocyte  and  the  primary  oocyte;  C,  secondary  spermatocyte  and  secondary 
oocyte;  Z>,  mature  egg  and  three  polar  bodies,  and  four  sperms;  E  and  F,  sperm 
and  egg  unite  and  the  nuclei  fuse;  G,  the  oosperm  nucleus  divides;  H  and  /,  the 
cell   divides.      A-D,    maturation    stages;   E   and   F,    fertilization;    G-I,   first 
cell  division.     In  F-G  the  two  cells  are  completely  fused.     Three  of  the  sperms 
and  the  three  polar  bodies  are  not  represented  after  stage  E. 


326 


GENERAL   PRINCIPLES 


• 

< 


FIG.  187. — Six  stages  in  the  process  of  fertilization  of  the  ovum  of  a  mouse. 
After  the  first  stage  figured  it  is  impossible  to  determine  which  of  the  two  nuclei 
represents  the  male  or  female  pronucleus.  ek.  Female  pronucleus;  rk\  and  rk*, 
polar  globules;  spk,  male  pronucleus.  (McMurrich  from  Sobotta.) 


CLEAVAGE  327 

ism,  whether  it  is  a  unicellular  form  or  whether  it  is  a  higher 
plant  or  animal.     Only  a  few  typical  cases  will  be  outlined. 

704.  (i)  When  the  cell  has  divided  the  daughter  cells  sepa- 
rate completely.     On  future  divisions  the  same  thing  occurs 
and  all  the  cells  remain  alike.     This  type  includes  the  free 
protozoa  and  many  unicellular  plants.     Sometimes  a  number 
of  divisions  occur  before  the  cells  separate,  so  that  a  group  of 
eight,  sixteen,  etc.,  cells  are  produced.     These  then  all  become 
free  at  once  and  become  independent  organisms. 

705.  (2)  The  cells  divide,  but  are  held  together  by  a  gelatin- 
ous matter  which  they  secrete  (Fig.  60) ;  or  by  their  cell  walls 
(Figs.  182  and  212),  or  by  a  connecting  bridge  which  develops 
into  a  branching  stalk.     These  are  colonies,  and  the  arrange- 
ment of  the  individuals  in  the  colony  depends  largely  on  whether 
the  planes  of  division  are  always  parallel,  forming  filaments 
(Figs.  182  and  212),  or  at  right  angles  in  two  planes,  form- 
ing plates,  or  at  right  angles  in  three  planes,  forming  cubical 
masses.     (Fig.  60.) 

706.  (3)  The  cells  divide  structurally,  but  remain  in  func- 
tional unity.     The  resulting  entity  is  not  a  colony,  but  still  re- 
mains a  single  organism.     Because  of  cell  division  the  nuclear 
matter  is  distributed  throughout  the  body  of  the  organism,  and 
the  size  of  the  body  is  not  limited  by  the  limited  distance 
through  which  the  nucleus  acts  on  the  cytoplasm,  as  would 
probably  be  the  case  if  there  were  but  a  single  central  nucleus. 
The  division  of  the  body  into  cell  units  also  permits  differen- 
tiation to  an  unlimited  degree,  and  with  it  division  of  labor 
and  perfection  of  function.     (See  pp.  136,  338.) 

707.  Under  this  head  two  types  of  development  occur  which 
we  may  characterize  as  evolving  and  involving.     In  the  former 
the  cell  mass  is  solid  and  leads  to  the  diffuse,  plant  type  of 
organization,  while  in  the  latter  the  cells  are  arranged  in  layers 
enclosing  cavities,  and  through  it  the  compact  animal  type  of 
organization  is  attained.     (See  pp.  328,  331  ff.)     This  apparent 


328 


GENERAL   PRINCIPLES 


paradox  disappears  when  we  recognize  that  the  " cavities" 
mentioned  are  not  so  much  spaces  as  cleavage  planes  between 
organs,  which  permit  the  involution  (growing  inward)  of  other 
organs. 

708.  As  examples  of  the  first  type  we  may  take  a  fern  and  a 
dicotyledon.  The  fertilized  egg  cell  of  the  fern  divides  into 
two  cells,  and  these  divide  again,  making  four.  These  four 
cells  continue  to  divide  indefinitely,  and  the  cells  remain  a 


FIG.  188. 


FIG.  189. 


FIG.  188. — Development  of  the  fern  embryo.  A,  The  egg  cell  divided  into 
quadrants;  B,  a  later  stage,  the  four  quadrants  still  evident  and  from  them 
develop  the  four  parts  of  the  plant  as  indicated.  /,  Develops  the  foot;  r,  develops 
the  root;  s,  develops  the  stem;  /,  develops  the  first  leaf. 

FIG.  189. — Development  of  the  dicotyledon  embryo,  i  and  2,  Early  stages 
showing  all  of  the  suspensor;  3  and  4,  only  the  end  of  the  suspensor  is  shown. 
In  2  the  embryo  is  marked  off  at  the  upper  end  of  the  suspensor.  In  3  the  em- 
bryo is  farther  advanced.  In  4  the  root  (T),  stem  (5)  and  cotyledons  (C)  are 
distinguishable. 


single  solid  mass.  From  each  quadrant  of  the  four-cell  stage 
a  definite  part  of  the  young  plant  develops;  that  is,  from  I 
develops  the  root,  from  II  the  stem,  from  III  the  first  leaf,  and 
from  IV  the  foot,  an  organ  by  which  the  plantlet  continues  to 
draw  nourishment  from  the  mother  prothallus.  This  foot  is  a 
type  of  structure  which  is  very  common  among  plants  and  ani- 
mals; that  is,  of  structures  which  are  functional  only  in  the 


CLEAVAGE  329 

embryonic  period  of  the  organism,  and  later  disappear  or  remain 
only  as  rudiments. 

709.  In  the  dicotyledon  embryo  a  similar  embryonic  structure 
is  found.     The  suspensor  is  a  row  of  cells  which  forms  no 
part  of  the  young  plant.     The  terminal  cell  divides  into  two, 
four,  eight,  etc.,  cells,  which  at  first  form  a  spherical  mass. 
This  soon  becomes  heart-shaped  and  then  the  four  parts  of  the 
dicotyledon  embryo  may  be  localized,  viz. :     I  the  caulicle,  II 
the  epicotyl,  and  III  and  IV  the  cotyledons. 

710.  In  many  of  the  higher  cryptogams  the  new  cells  are  all 
divided  off  from  one  terminal  or  apical  cell.     In  some  cases  a 
few  divisions  may  occur  in  segments  cut  off  from  the  apical 
cell,  but  only  the  apical  cell  is  capable  of  continued  division. 
In  the  dicotyledon,  cell  multiplication  takes  place  only  in  limited 
regions  of  the  bud  and  root-tip  and  in  the  cambium.     These 
have   already   been   described    (p.   48).     (Growth   in    Mono- 
cotyledons, see  p.  343.) 

711.  Types  of  Cleavage. — The  course  of  segmentation  of 
animal  eggs  is  greatly  modified  by  the  quantity  of  yolk  present 
in  the  egg,  and  in  this  respect  animal  eggs  vary  greatly.     Some 
eggs  contain  little  yolk  (Ccelenterates,  Worms,  Echinoderms, 
the  lancelet,  Mammals),  others  have  moderate  quantities  (many 
Molluscs,  some  Fishes,  Amphibia),  while  in  still  others  the 
quantity  of  yolk  may  be  many  times  in  excess  of  the  protoplasm 
(Arthropods,  Cephalopods,  many  Fishes,  Reptiles  and  Birds). 
The  effect  of  the  yolk  is  to  retard  or  even  completely  inhibit 
division.     The  simplest  type  of  development  occurs,  therefore, 
in  eggs  with  little  yolk,  and  as  an  example  of  this  type  we  will 
take  the  egg  of  the  lancelet. 

712.  The  Blastula. — The  first  cleavage  plane  of  the  egg  of 
the  lancelet  is  vertical,  the  second  is  also  vertical  and  at  right 
angle  to  the  first.     The  third  is  horizontal  and  cuts  the  four  seg- 
ments slightly  above  the  centres,  so  that  the  four  lower  cells  are 
slightly  larger  than  the  four  upper.     This  is  due  to  the  slightly 


330 


GENERAL   PRINCIPLES 


(Figures  190  to  210  represent  the  early  steps  in  the  development  of  Amphioxous 
Branchiostoma).     According  to  the  wax  models  of  Ziegler,  Freiburg,  i.  B. 


FIG.  190. — The  mature  egg  with 
the  polar  body  above. 


FIG.  191. — Two-cell  stage. 


FIG.  192. — Four-cell  stage. 


FIG.  193. — Eight-cell  stage. 


FIG.  194. — Sixteen-cell  stage.        FIG.  195. — Thirty-two  cell  stage,  in  section. 


BLASTULA,   GASTRULA 


33 1 


larger  quantity  of  yolk  at  the  lower  pole  of  the  egg.  For  the 
same  reason  the  succeeding  divisions  take  place  slightly  more 
rapidly  at^the  upper^pole  of  the  egg,  and  hence  the  cells 


FIG.  196. — Blastula. 


FIG.  197. — Blastula,  later  stage. 


FIG.  198. — Gastrulation. 


FIG.  199. — Gastrulation,  later  stage. 


here  are  also  somewhat  smaller.  The  cells  have  a  decided 
tendency  to  round  up  after  division  and  so  give  the  whole 
somewhat  the  appearance  of  a  mulberry.  Hence,  this  is  called 


332 


GENERAL  PRINCIPLES 


FIG.  200. 


FIG.  201. 


FIG.  202. 


FIG.  203. 


FIGS.  200  to  203  represent  four  steps  in  the  development  of  the  larva  from  the 
gastrula.  The  gastrula  mouth  narrows  and  is  finally  closed  in  by  the  ectoderm. 
The  gastrula  cavity  elongates  and  becomes  the  primitive  digestive  cavity 
(archenteron). 


THE   EMBRYO  333 

the  mulberry  or  morula  stage.  In  the  centre  of  the  mass  a 
space  is  formed  between  the  rounded  inner  surfaces  of  the  cells, 
and  this  grows  larger  as  division  proceeds.  The  free  sur- 
faces of  the  cells  become  flatter  and  they  adhere  more  closely 
to  their  neighbors.  By  this  way  we  arrive  at  a  stage  called  the 
blastula,  a  hollow  sphere,  the  wall  of  which  is  formed  by  a 
single  layer  of  cells.  The  cavity  of  the  sphere  is  the  blastula 
cavity. 

713.  The  Gastrula.— By  the  next  series  of  changes  the  blas- 
tula is  transformed  into  a  gastrula.     Cell  division  continues 
and  the  embryo  increases  in  size,  but  we  will  fix  our  attention 
on  another  set  of  changes:     First,  the  vegetative  (yolk)  pole 
of  the  blastula  becomes  flattened,  then  concave,  as  seen  from 
the  outside.     This  cavity  deepens  and  thereby  the  blastula 
cavity  grows  smaller,  until  it  is  finally  obliterated,  when  the 
inverted  vegetative  hemisphere  comes  in  contact  with  the  hem- 
isphere of  the  animal  pole.     The  embryo  now  has  the  form  of 
a  cup,  with  double  walls.     This  stage  is  the  gastrula.     The 
new   cavity   which  has  been  formed  is   the  gastrula  cavity 
(archenteron),  and  its  opening  to  the  exterior  is  the  gastrula 
mouth.     The  two  layers  which  form  the  wall  of  the  gastrula 
are  the  ectoderm  and  entoderm.     After  a  time  the  embryo  has 
elongated,  and  by  unequal  growth  its  axis  has  been  shifted  so 
that  now  the  gastrula  mouth,  which  has  become  very  small, 
lies  at  the  posterior  dorsal  extremity. 

714.  The  Medullary  Plate. — From  now  on  several  important 
developmental   processes   occur   simultaneously,   but   we   will 
trace  them  one  by  one. 

715.  The  dorsal  surface  of  the  elongated  embryo  becomes 
flattened  and  the  cells  of  the  ectoderm  along  the  median  line 
assume  a  columnar  form,  which  results  in  a  thickening  of  the 
ectoderm.     The  medullary  plate  thus  formed  curls  up  along 
its  edges,  forming  a  medullary  groove.     The  edges  of  the  groove 
approach  above  and  fuse  to  form  the  medullary  tube.     This 


334 


GENERAL    PRINCIPLES 


FIG.  204.  FIG.  205. 

FIG.  204  is  a  cross  section  of  the  stage  represented  in  Fig.  203.  The  medullary 
plate  is  shown  above,  between  entoderm  and  ectoderm. 

FIG.  205.— A  later  larval  stage  showing  six  mesodermic  pockets,  and  the 
medullary  plate  completely  covered  by  the  ectoderm. 


FIG.  206.  FIG.  207. 

FIG.  206  is  a  cross  section  through  the  anterior  (left  in  the  figure)  end  of  the 
stage  represented  by  Fig.  205. 

FIG.  207  is  a  similar  section  through  the  posterior  end  of  the  same  stage.  In 
both  figures  the  medullary  groove,  the  mesodermic  somites  and  the  first  steps 
in  the  formation  of  the  notochord  are  shown. 


THE   EMBRYO 


335 


FIG.  208.  FIG.  209. 

FIG.  208  is  a  horizontal  section  through  the  stage  represented  by  Fig.  205, 
and  shows  the  six  pairs  of  mesodermic  somites. 

FIG.  209  is  a  section  through  a  later  stage  (Fig.  210)  and  shows  the  medullary 
tube,  the  notochord,  and  the  mesoderm  pushing  upward  and  downward  between 
entoderm  and  ectoderm. 


FIG.  210. — A  later  stage,  showing  most  of  the  mesodermic  somites  completely 
cut  off  from  the  entoderm,  the  notochord,   and  the  medullary  tube. 


336  GENERAL  PRINCIPLES 

tube  ultimately  gives  rise  to  the  central  nervous  system  of  the 
animal.  Before  the  tube  is  completely  formed  the  edges  of  the 
ectoderm  slip  over  it  from  each  side  toward  the  median  line 
and  fuse.  This  superficial  part  of  the  ectoderm  gives  origin 
to  the  epidermis  with  its  modifications,  and  the  sense  organs  of 
the  skin. 

716.  The  Notochord. — While  the  medullary  plate  is  form- 
ing in  the  ectoderm  the  notochord  and  mesoderm  are  also 
taking   origin  from   the   entoderm.     A   longitudinal  inverted 
groove  is  formed  by  the  bulging  upward  of  the  entoderm  along 
the  mid-dorsal  line.     The  edges  of  this  groove  unite  to  form  a 
tube,  which  is  then  cut  off  from  the  entoderm.     This  tube 
develops  into  the  notochord. 

717.  The  Mesoderm. — At  the  same  time  two  series  of  pock- 
ets are  formed  by  the  bulging  outward  of  the  entoderm  in  the 
dorsal  lateral  quarters,  i.  e.,  on  either  side  of  the  notochord. 
These  pockets  also  close  and  are  cut  off  from  the  entoderm. 
They  are  called  mesodermic  somites,  and  are  the  first  evidence 
of  the  segmentation  of  the  body.     They  gradually  extend  down- 
ward and  upward  till  they  finally  completely  surround  the 
medullary  tube,  the  notochord  and  the  remaining  entoderm. 
The  cavities  of  the  mesodermic  pockets  develop  into  the  body 
cavity. 

718.  Other  Types  of  Cleavage. — So  far  as  the  segmentation 
stages  are  concerned  the  chief  deviations  from  the  lancelet 
type  may  be  ascribed  to  the  quantity  and  disposition  of  the 
yolk.     In  the  lancelet  egg,  cleavage  is  total  and  equal.     In  the 
frog's  egg  there  is  a  large  amount  of  yolk  accumulated  largely 
at  the  vegetative  pole.     In  consequence  cleavage,  though  also 
total,  is  unequal  and  at  the  first  horizontal  division  the  cells  of 
the  vegetative  pole  are  many  times  larger  than  those  of  the  ani- 
mal pole.     In  the  eggs  of  Cephalopods,  many  Fishes,  Reptiles 
and  Birds,  the  quantity  of  protoplasm  is  small  compared  with 
the  yolk  and  forms  a  thin  layer  at  the  animal  pole.     When 


TYPES   OF   CLEAVAGE  337 

cleavage  takes  place  the  yolk  does  not  divide  and  the  cleavage 
only  extends  through  the  protoplasm,  which  is  thus  divided 
into  a  number  of  minute  hummocks.  From  these,  cells  are 
later  cut  off  by  horizontal  cleavage  planes,  and  the  same  process 
extends  outward  and  downward,  continuously  adding  to  the 
number  of  fully  formed  cells.  This  type  of  cleavage  is  called 
partial  and  discoidal,  the  latter  referring  to  the  disk-like  form 
of  the  segmenting  area.  For  most  Arthropods  cleavage  follows 
still  another  course.  Here  the  eggs  are  also  laden  with  yolk, 
which  is  concentrated  at  the  centre  of  the  egg,  while  the  proto- 
plasm lies  more  equally  distributed  over  the  entire  surface. 
When  the  nucleus  divides  there  is  at  first  no  division  of  the 
cytoplasm.  But  after  a  time  the  nuclei  arrange  themselves 
near  the  surface  in  a  single  layer.  The  cytoplasm  then  divides 
partially  as  in  the  preceding  case,  but  over  the  whole  surface 
of  the  egg.  This  type  is  distinguished  as  partial  and  superficial 
cleavage. 

719.  Origin  of  the  Tissues.— If  allowance  is  made  for  the 
modification  caused  by  the  yolk,  one  may  say  that  during  the 
early  stages  of  development  all  metazoa  proceed  along  parallel 
lines.  In  all  cases  a  blastula  is  formed,  and  this  is  followed  by 
a  gastrula  stage.  The  formation  of  a  distinct  mesoderm,  how- 
ever, does  not  occur  in  the  Porifera  and  Ccelenterates.  These 
animals,  even  in  the  adult,  consist  only  of  two  distinct  cell 
layers,  the  ectoderm  and  entoderm,  though  there  is  usually  a 
supporting  layer  between.  This  may  be  simply  a  structureless 
membrane  or,  in  some  cases,  a  thick  layer  composed  chiefly  of 
'5  gelatinous  matrix.  Some  of  the  ectodermal  and  entodermal 
cells  may  send  nervous  or  muscular  fibre  processes  into  this 
layer,  and  there  may  also  be  few  or  many  cells  of  ectodermal  or 
entodermal  origin  completely  embedded  in  this  layer.  In  the 
other  phyla,  where  a  true  mesoderm  is  formed,  there  is  consider- 
able variation  in  the  method.  It  is  almost  always  wholly  ento- 
dermal in  origin,  but  some  times  it  begins  as  a  solid  outgrowth, 


33 8  GENERAL   PRINCIPLES 

which  later  splits  to  form  the  body  cavity,  and  again  it  is  gradu 
ally  formed  by  cells  which  sink  inward,  one  by  one,  from  the 
entoderm  and  finally  arrange  themselves  around  the  bod} 
cavity. 

720.  In  general,  after  the  mesoderm  and  notochord  have  beer 
formed,  the  entoderm  which  remains  forms  the  lining  of  th( 
digestive  tract;  that  is,  the  mucous  epithelium  with  all  th( 
glandular  organs  connected  with  it,  salivary,  thymus,  thyroid 
gastric,  hepatic,  pancreatic  and  intestinal  glands.     The  mucous 
epithelium  lining  the  tracheae,  bronchi  and  lungs,  is  also  oJ 
entodermal  origin. 

721.  The  mesoderm  gives  rise  to  all  other  parts  of  the  body 
the  dermis,  muscles,  all  connective  tissues,  including  bones 
cartilage,  teeth  (in  part),  ligaments,  tendons,  fascia;  heart 
blood  vessels  and  blood;  lymph,  lymph  glands,  and  spleen;  the 
gonads  and  kidneys  with  their  ducts;  the  pleura,  pericardium, 
peritoneum  and  mesenteries. 

722.  Indirect  Development. — The   course  pursued  by  the 
developing  organism  is  not  always  the  most  direct.     When 
the  embryo  gradually  assumes  the  characters  of  the  adult  the 
development  is  said  to  be  direct.     Often,  however,  there  is  a 
sudden  change  in  direction  of  development,  so  that  the  earlier 
steps  seem  to  be  directed  toward  a  very  different  goal  from  that 
finally  reached;  so,  for  example,  in  most  forms  which  are  fixed, 
or  very  sluggish  in  the  adult,  there  is  an  active  free-swimming 
larva  (Echinoderms,  most  Molluscs,  Worms,  Barnacles).     This 
is  called  a  dispersal  larva  because  it  seems  to  distribute  the  spe- 
cies partly  by  its  own  efforts,  but  more  largely  by  the  currents 
by  which  it  is  carried  about.     An  interesting  exception,  which 
proves  the  rule,  is  that  of  the  larvae  of  the  fresh-water  clam. 
If  this  were  to  follow  the  rule  the  larvae  would  be  carried  down 
stream  by  the  currents  much  farther  than  the  adult  would  be 
able  to  move  upward  during  its  entire  existence.     The  result 
would  be  that  the  entire  species  would  soon  be  carried  to  the 


INDIRECT   DEVELOPMENT 


339 


sea.     But  this  larvae  attaches  itself  to  the  gills  of  fishes  and  may 
thus  be  carried  up  stream. 

723.  Another  type  of  larva  is  the  trophic  larva,  like  those  of 
many  insects.  These  are  characterized  by  their  voracious 
appetites.  The  caterpillar,  for  example,  consumes  much  more 
than  is  necessary  for  its  daily  needs.  The  excess  is  stored  up 
in  the  tissues  as  fat.  When  a  certain  stage  of  development  is 
reached,  feeding  ceases.  The  caterpillar  finds  a  suitable  place 


FIG.  211. — Development  of  the  frog.  An  example  of  indirect  development 
(metamorphosis).  The  gills  have  not  disappeared  in  stage  6  but  have  been 
covered  by  a  fold  of  the  skin.  (From  Galloway  after  Brehm.) 

in  which  to  rest.  Here  a  complete  change  takes  place.  The 
skin  is  cast  and  there  emerges  a  quiescent  pupa,  without  func- 
tional appendages,  without  mouth  or  eyes.  In  this  form  the 
animal  remains  for  a  shorter  or  longer  time,  a  few  days  to  many 
months.  Then  another  radical  change  occurs.  The  pupa  skin 
is  cast  and  the  winged  adult  emerges.  In  many  cases  the  adult 
feeds  very  little.  Some  have  no  functional  mouth  parts  and 
would  be  unable  to  take  food.  They  live  for  a  short  time,  mate, 
and  deposit  eggs.  Here  the  adult  is  the  dispersal  stage.  Its 
organs  of  locomotion,  the  wings,  are  not  for  the  ordinary  pur- 


340  GENERAL   PRINCIPLES 

poses  of  securing  food  or  escaping  enemies  as  much  as  to 
facilitate  mating  and  the  carrying  of  the  species  to  new  and 
favorable  localities. 

724.  Differentiation  of  Germinal  and  Somatic  Tissues.— 
Reproduction  in  the  protozoa  and  most  unicellular  plants  (all 
unicellular  organisms  are  some  times  grouped  together  under 
the  name  Protista)  means  merely  a  division  of  the  body  of  the 
organism.  The  two  halves  resulting  from  division  reorganize 
themselves,  and  after  a  period  of  growth,  each  has  attained  to 
the  condition  of  the  parent  cell  before  division.  This  process 
continues  indefinitely,  and  apparently  there  is  no  inherent 
reason  why  the  substance  of  the  Protist  should  not  continue 
thus  indefinitely;  that  is,  the  Protist  organism  does  not  naturally 
end  in  death.  With  the  metazoan  the  case  is  entirely  differ- 
ent. At  a  certain  stage  of  development  the  body  is  divided 
into  two  classes  of  cells.  A  relatively  small  portion  consists 
of  cells  destined  to  develop  into  gametes  and,  therefore,  to  con- 
tinue on  in  the  succeeding  generation.  All  the  remaining  cells 
of  the  body  come  to  an  end  with  the  death  of  the  individual. 
These  two  types  of  cells  are  distinguished  as  germ  cells  and 
somatic  cells.  This  distinction  rests  on  the  principle  of  divi- 
sion of  labor.  Here,  as  everywhere  else  in  the  biological  world, 
the  chief  end  is  the  perpetuation  of  the  species.  This  is 
secured  more  certainly  if  by  unity  of  action  but  division  of 
labor  the  life  functions  are  performed  in  the  most  perfect 
manner.  This  seems  to  be  the  reason  for  the  existence  of  the 
multicellular  organism. 

727.  Division  of  Labor  and  Differentiation.— All  the  various 
types  of  aggregation  of  biological  units  are  attempts  to  solve 
the  same  problem.  The  colonies  of  Protists,  the  hydroid 
colonies  with  polymorphism,  the  polymorphic  societies  of  ants 
and  bees,  the  various  types  of  alternation  of  generation  found 
among  plants  and  animals  are  all  devices  to  secure  the  most 
perfect  functioning  by  dividing  the  functions.  The  meta- 


GROWTH  341 

zoan  individual  is  in  one  sense  a  colony  of  cells  in  which  there 
is  unity  of  action.  But  in  order  to  secure  the  best  results,  the 
division  of  labor  means  differentiation,  and  differentiation 
carried  very  far  means  loss  of  power  of  recuperation  and  of  cell 
division.  Thus  the  functions  of  the  somatic  cells  contribute 
to  the  perpetuation  of  the  race  by  fostering  the  gametes  and 
further  nursing  the  embryo  in  many  cases  until  it  is  well  ad- 
vanced in  development. 

726.  Regeneration. — That  the  loss  of  the  power  of  recupera- 
tion goes  hand  in  hand  with  differentiation,  appears  from  a 
study  of  the  phenomena  of  regeneration.     An  egg  has  the  power 
of  regeneration  like  that  of  a  protozoan,  so  that  from  a  fragment 
of  an  egg  a  complete  embryo  may  develop.     This  is  true  even 
of  Vertebrate  eggs.     Among  the  lower  invertebrates  such  power 
of  regeneration  exists  even  in  the  adult.     An  anemone  or  star 
fish  may  be  cut  in  two,  and  the  parts  will  regenerate  all  the 
organs  that  were  removed.     This  is  not  possible  with  a  crab  or 
crayfish.     But  even  the  Crustaceae  will  regenerate  an  append- 
age that  was  broken  off.     So  as  we  pass  to  higher  forms  the 
power  regularly  decreases.     In  young  Amphibia,  appendages 
may  still  be  regenerated,  but  in  Birds  and  Mammals  the  power 
extends  only  to  the  healing  of  wounds,  which  is  also  a  regenera- 
tion process. 

727.  Mechanics  of  Growth. — Differentiation  begins  at  a  very 
early  stage.     Even  in  the  lancelet   the   entoderm   is    distin- 
guishable from  the  ectoderm  before  gastrulation  takes  place, 
and  the  cells  of  the  medullary  plate  are  distinguishable  from 
the  rest  of  the  ectoderm  before  the  medullary  groove  is  formed. 
But  it  will  not  be  necessary  to  describe  the  individual  types  of 
tissues  here,  since  this  was  done  in  connection  with  the  anatom- 
ical description  of  types.     There  are,  however,  some  points 
about"*  the  mechanics  of  growth  which  should  be  noted.     A 
naked  ~ protoplasmic    cell   like   Amoeba   expands   freely   with 
growth_and  often  when  there  is  a  cell  membrane  it  is  elastic 


342 


GENERAL   PRINCIPLES 


enough  to  expand,  with  the  growth  of  its  contents.  But  there 
are  often  rigid,  supporting  or  protecting  structures,  which  do 
not  permit  free  expansion  of  the  body.  In  such  cases  there  are 

many  interesting  devices  employed 
for  securing  expansion.  The  split- 
ting bark  of  the  exogen  type  of  stem 
has  been  described,  as  well  as  the 
growing  root-tip  and  the  bud.  If 
the  position  of  the  apical  cell  and  the 
angles  of  the  planes  in  which  the  suc- 
cessive segments  are  cut  off  from  it 
are  carefully  considered  it  will  be 

OU    U  *         seen  that  we  have  here  also  a  device 

1      \*\      L-4        for  securing  freedom  for  growth.     Let 
0Vl  KM      I  —  I  k     us  consider  a  ^ew  more  cases  among 
Ll     ll      I    I        plants.      The    diatoms    are    always 
unicellular,  -and  each  cell  is  encased 
in  a  silicious  capsule.     The  substance 
of  which  this  capsule  is  composed  is 
absolutely  unyielding  so  far  as  the 
growth  of  the  living  contents  is  con- 
cerned.    But  the  capsule  is  composed 
of  two  parts,  which  fit  into  each  other 


FIG.  212. — Mechanics  of 
growth.  A,  A  diatom;  B, 
Microspora;  C,  (Edogonium. 
In  A:  a,  the  silicious  " pill-box" 
shell  of  a  diatom;  b,  a  diatom 
dividing  and  forming  two  new 
half  shells,  back-to-back,  within 
the  old  one.  In  B:  a,  b  and  c, 


three   steps  in   the  process  of     ri        .-,  r  i    ,. 

forming  a  new  cross  wall  and     llke  the  Parts  of  a   Common  gelatin 

capsule  or  a  pill  box,  and  can  slide 
apart  as  the  protoplasmic  contents 
increase  in  volume.  At  the  time  of 


elongating  the  side  walls  of  a 
dividing  cell.  In  C:  a,  the  cir- 
cular pad  formed  within  the  old 
wall  preparatory  to  elongation; 
b,  the  old  wall  split  under  the 
pad  and  the  pad  stretching  to 
form  new  wall;  c,  ridges  left  by 
a  repetition  of  the  process. 


division  a  new  half  capsule  is  formed 
inside  each  of  the  old  half  capsules. 
This  means  that  each  generation  is 

confined  in  a  slightly  smaller  compass  than  the  preceding. 
Finally  a  limit  is  reached  beyond  which  this  decreasing  size 
will  not  go.  The  shell  is  then  cast  off  completely,  a  brief 
period  of  rapid  growth  as  a  naked  cell  ensues,  and  then  a 


GROWTH  343 

new   and  larger  shell  is  produced  as  a  starting  point  for  a 
repetition  of  the  process. 

728.  Sometimes  cells  adhering  in  filaments  are  encased  in 
thick  tubular  sheaths,  which  become  too  unyielding  to  keep 
pace  with  the  growing  contents  by  stretching.     In  the  case  of 
Tolyptothrix  the  tube  splits  at  a  certain  point,  the  chain  of 
cells  breaks,  and  one  end  pushes  past  the  other  and  out  through 
the  opening.     This  produces  what  is  called  false   branching. 
Again  in  Oedogonium  the  unyielding  tube  in  which  the  cells 
are  encased  is  made  to  expand  in  a  curious  fashion.     Inside 
the  cell  a  circular  cushion  of  new  cell  wall  substance  is  formed 
against  the  inner  surface  of  the  old  wall.     This  cushion  com- 
pletely encircles  the  cylinder.    Then  the  old  wall  breaks  opposite 
the  cushion,  and  this  permits  the  latter  to  stretch  and  the  cell 
to  elongate,  while  at  the  same  time  maintaining  the  continuity 
of  the  cell  wall.     In  Microspora  the  wall  of  the  filament  is  made 
up  of  segments  which  are  double-wedge-shaped  in  longitudinal 
section.     These  slip  apart  as  growth  proceeds  and  permit  the 
insertion  of  new  wedges  by  growth. 

729.  Among  the  Monocotyledons  two  methods  of  common 
occurrence  are  of  special  interest.     In  this  group  of  plants  we 
find  the  sheathing  leaf  base  a  very  common  type.     The  sheath 
completely  surrounds  and  protects  the  stem  for  some  distance 
upward  from  the  node.     Within  this  sheath  the  stem  remains 
for  a  long  time  meristematic,  after  the  upper  part  of  the  same 
internode  has  completed  its  growth.     This  device  permits  the 
stem  to  continue  growth  longitudinally  for  a  long  time,  but 
growth  in  thickness  is  practically  completed  when  the  upper 
end  of  the  node  appears  above  its  sheath.     This  type  is  espe- 
cially characteristic  of  the  long,  slender,  rapidly  growing  stems 
of  grasses.     A  modification  of  this  type  occurs  in  the  case  of 
the   perennial    Monocotyledons    like   the  palms.     Here,   the 
growth  in  height  is  very  slow,  and  the  short  internodes  are  pro- 
tected for  a  long  time  by  the  basal  portions  of  the  leaf  stalks, 


344 


GENERAL   PRINCIPLES 


which  often  develop  a  very  elaborate  protective  tissue.     This  | 
often  persists  long  after  the  death  of  the  leaf  as  a  thick  inter-  | 
woven  niat  of  tough  fibres.     Protected  in  this  way  the  stem  ! 
slowly  increases  in  thickness  for  several  years.     When  the  pro- 
tective tissue  finally  rots  away  and  exposes  the  stem  the  tissues  j 

of  the  latter  have  reached  their  i 
final  condition  and  the  stem 
no  longer  expands  in  diameter. 
Thus  the  stem  of  this  type 
soon  reaches  a  limit  in  diameter 
while  the  growth  in  height 
may  continue  indefinitely,  a 
condition  which  is  decidedly 
inferior  to  that  of  the  exogen 
stem,  in  which  growth  in  thick- 
ness keeps  pace  with  growth 
in  height. 

730.  For  animals,  the  ques- 
tion of  growth  mechanics  is 
fundamentally  not  as  difficult 
as  for  plants,  because  the  tis- 
sues are  generally  more  yield- 
ing in  character.  At  the  same 
time  the  problem  has  appeared 
in  much  greater  variety  and 

FIG.  213.— Section  of  a  conch  shell   has  been  solved  in  more  differ- 

entways.    As  soon  as  animals  . 
covered     themselves    with    a 

protecting  shell  they  learned  the  trick  of  making  that  shell 
conical  in  form,  so  that  by  adding  to  the  edge  or  mouth  of  the 
cone  it  grew  wider  as  well  as  longer.  This  type  of  shell  is 
found  in  many  forms,  from  the  protozoa  to  the  cephalopods. 
But  a  flat  cone  offers  less  protection,  while  a  long  one  is 
awkward  to  handle.  This  difficulty  was  also  soon  solved  by 


ECDYSIS  345 

coiling  the  cone  into  a  spiral  by  more  rapid  growth  on  one 
side.  This  device  is  also  very  generally  employed  wherever 
the  cone  is  in  use  (many  Protozoa,  some  Worms,  Gastropods, 
Cephalopods,  especially  extinct  forms). 

731.  One  of  the  most  distinctive  characters  of  the  entire 
phylum  of  Arthropods  is  the  way  the  problem  of  growth  me- 
chanics  is  solved.     The  Arthropod  is  entirely  enclosed  in  a 
sheathing  of  chitin,  a  substance  which  is  very  elastic  but  has  very 
little  power  of  stretching.    In  fact,  the  animal  cannot  grow  while 
encased  in  this  armor.    Consequently,  the  armor  is  removed  peri- 
odically and  then  a  period  of  rapid  expansion  ensues  until  the 
new  shell  has  hardened  again.    Among  the  Crustaceae  the  casting 
of  the  shell  (ecdysis)  occurs  frequently  during  the  early  periods 
of  development  (lobster  7-8  times  in  first  year),  later  the  moult- 
ing periods  are  less  frequent  (once  per  year,  crab,  lobster). 
During  the  soft-shell  periods  the  animal  remains  concealed  in 
some  cranny,  because  it  is  then  extremely  helpless.     Not  only 
is  it  unprotected  by  a  shell,  but  its  "  claws, "  at  other  times  so 
formidable,  are  now  useless.     Nevertheless,  the  Crustaceae  as  a 
class  have  been  very  successful,  and  we  must  conclude  that  the 
disadvantages  of  the  period  of  ecdysis  are  more  than  compen- 
sated by  the  advantages  of  the  chitinous  armor. 

732.  The  more  primitive  Insects  follow  in  general  the  Crus- 
taceae in  regard  to  the  management  of  this  armor,  but  most 
orders  of  Insects  have  adopted  a  different  and  probably  a  better 
plan.     Diptera,  Coleoptera,  Hymenoptera  and  Lepidoptera,  the 
most   numerous    orders,    develop   by   metamorphosis.     Their 
larvae  are  soft-skinned  and  are  in  various  ways  enabled  to   dis- 
pense with  the  armor.     During  the  pupal  stage  they  are  usually 
concealed  in  the  earth  or  elsewhere,  and  after  they  emerge  as 
completely  armored  insects,  they  no  longer  grow.     Their  growth 
is  completed  and  no  ecdysis  is  needed.     Ecdysis  occurs  at  the 
period  of  pupation,  and  again  at  the  emergence  of  the  imago, 
and  at  this  time  the  insect  is  often  concealed. 


346 


GENERAL  PRINCIPLES 


733.  The  development  of  an  internal  skeleton  by  the  Verte- 
brates called  for  a  new  solution  of  these  growth  problems.  But 
in  the  Vertebrates  numerous  integumentary  structures,  each 
with  its  peculiar  method  of  growth,  are  also  found.  We  will 
only  consider  the  epidermis  here.  This  layer  of  the  skin  is  con- 
stantly growing  in  all  the  terrestrial  Vertebrates  and  its  dead, 


FIG.  214. — Ecdysis  of  the  blue  crab.  The  animal  (lower  part  of  the  figure) 
has  almost  freed  itself  of  the  shell  from  which  it  escapes  by  backing  out. 
Xi/2. 

outer  layers  are  cast  either  at  some  intervals  of  time  or  as  a  con- 
tinuous process.  In  the  Amphibia,  this  layer  is  cast  at  intervals 
in  large  or  small  patches.  In  Snakes  it  often  comes  away  in  a 
single  piece.  In  Birds  and  Mammals  the  epidermis  is  constantly 
shedding  in  minute  scales,  but  in  these  two  classes  there  is 
usually  a  well-marked  period  of  moulting  or  shedding  of  feathers 
and  hair.  When  the  epidermis  is  cast  it  is,  of  course,  not  the 


GROWTH 


347 


entire  layer,  nor  even  all  the  dead  tissue.  Only  the  hardened, 
more  superficial  part  separates  from  the  deeper,  more  flexible, 
layers.  In  some  cases  (Reptiles)  a  specially  constructed  layer 
of  cells  forms  a  cleavage  plane.  The  process  is  comparable  to 
the  ecdysis  in  Arthropods,  except  that  here  we  have  to  do  with 
dead  cells  instead  of  formed  substances. 

734.  The  method  of  growth  of  the  bones  varies  greatly.     In 
the  smaller  bones  with  simple  form,  the  process  is  not  specially 


FIG.  215. — The  carapace  of  the  diamond  back  terrapin,  Malaclemmys  palus- 
tris.     Note  the  concentric  lines  of  growth  in  the  horny  plates.     X 1/2. 

noteworthy,  but  with  the  "long"  bones  and  those  of  com- 
plicated figure,  the  enlargement  of  the  bone,  and  at  the  same 
time  maintaining  its  form,  is  often  a  complicated  process.  For 
example,  the  skull  of  the  adult  is  practically  a  single  piece. 
This  condition  could  not  have  been  reached  by  the  addition  of 
layers  of  bone  to  the  surface,  since  this  would  not  provide  for 
the  growth  of  the  brain  unless,  at  the  same  time,  the  cavity  of 


348  GENERAL   PRINCIPLES 

the  skull  were  enlarged  by  the  removal  of  material  from  the 
inner  surface  of  the  skull  bones.  The  end  is  accomplished  in 
another  way.  The  skull  is  composed  of  many  pieces,  which  are 
fitted  together  in  a  peculiar  way.  The  seam,  or  suture,  along 
which  two  bones  join  is  not  an  even  line  or  smooth  joint;  it  is  an 
extremely  sinuous  line  which  effects  a  dovetailing  of  the  two 


FIG.  216. — Skull  of  a  human  embryo  at  time  of  birth.  The  bones  are  still 
separated  by  seams  of  cartilage  and  membrane.  The  broad  unossified  space  is 
called  a  fontanelle.  In  the  figure  the  radiating  lines  on  the  parietal  bone  (large 
bone  on  the  left)  indicate  the  original  centre  of  ossification  and  the  direction  of 
growth. 

bones  in  a  way  to  produce  a  very  firm  joint.  The  suture  dis- 
appears at  maturity  by  the  complete  fusion  of  the  bones,  but 
until  the  end  of  the  growth  period  the  suture  is  an  open  joint,  in 
which  material  is  being  added  to  the  bones  of  both  sides.  The 
skull,  as  a  whole,  therefore,  expands  by  interstitial  growth,  while 


GROWTH 


349 


each  individual  bone  increases  its  dimensions  by  the  addition  of 
material  along  the  sutural  surfaces.  In  Reptiles  the  sutures 
tend  to  remain  open  throughout  life. 

735.  In  the  ossification  of  the  long  bones  like  the  femur,  the 
bone  is  first  deposited  on  the  surface  of  the  cartilage  in  its 


FIG.  217. — Cross  section  of  a  vertebra  of  an  embryo  (pig)  showing  centres  of 
ossification.  The  parts  in  black  are  cartilage.  At  three  points  the  cartilage  is 
being  replaced  by  bone;  in  the  centrum  (A)  and  in  the  two  sides  of  the  neural 
arch  (B).  As  the  bony  parts  grow  outward  into  the  cartilage  the  cartilage 
between  them  also  grows.  Thus  the  vertebra  increases  in  size  with  the  growth 
of  the  body.  Finally,  however,  all  the  cartilage  is  replaced  by  bone  and  the 
parts  unite  to  form  a  single  body  of  bone. 

middle  region.  The  two  ends  remain  cartilaginous  for  some  time 
and  grow  by  the  growth  of  the  cartilage.  In  the  middle  region 
or  shaft  the  growth  of  bone  continues  by  the  addition  of  new 
layers  to  the  outside,  and  the  new  layers  extend  gradually 


350 


GENERAL  PRINCIPLES 


GROWTH  351 

further  toward  the  two  ends.  After  a  time  new  centres  of 
ossification  occur  at  each  end.  These  form  bony  discs,  which 
are  later  separated  from  the  shaft  only  by  a  thin  seam  of  carti- 
lage. In  this  seam  there  are,  however,  three  zones  of  growth ;  a 
middle  zone,  in  which  cartilage  is  rapidly  forming,  and  on  either 
side  of  this  is  a  zone  in  which  the  cartilage  is  being  eroded  and 
replaced  by  bone.  It  is  thus  that  the  shaft  increases  in  length, 
while  the  epiphysis  is  also  increasing  in  thickness  (compare 
with  the  growth  in  the  cambium  ring) .  When  growth  is  com- 
plete the  epiphysis  unites  with  the  shaft. 

736.  This  method  by  which  a  complicated  skeletal  figure 
expands  by  interstitial  growth  through  the  growth  of  parts  is 
also  found  among  invertebrates.     An  excellent  example  is  in 
the  test  of  the  sea  urchin. 

737.  In  cases  where   the  proper  form  cannot  be  secured 
through  growth  by  the  addition  of  material  to  earlier  stages, 
it  often  happens  that  parts  of  the  earlier  structure  are  actually 
removed,  so  that  growth  consists  in  a  process  of  tearing  down 
and  building  larger.     This  takes  place,  for  instance,  in  many 
gastropod  shells  which  form  a  thick  rounded  fold  at  the  mouth 
of  the  shell  at  the  close  of  the  seasonal  growth  period.     At  the 
beginning  of  the  next  growing  season  this  fold  is  removed  by 
absorption  before  the  edge  of  the  shell  is  extended.     (Ex. :     The 
queen  conch.)     This  also  very  often  occurs  in  the  development 
of  the  Vertebrate  skeleton.     The  central  cavity  of  the  shaft  of 
an  adult  femur,  for  example,  is  much  larger  than  it  was  when 
the  first  layer  of  bone  was  laid  down  on  the  cartilage.     Hence, 
the  bone  must  have  been  removed  at  a  later  period.     So  also 
the  lower  jaw  of  the  embryo,  with  its  complex  curvature,  can- 
not be  included  within  the  outline  of  an  adult  jaw.     There 


FIG.  218. — Three  successive  steps  in  the  growth  of  the  queen  conch  (Cassia). 
The  thickened  lip  of  the  shell  (+)  in  A,  is  shown  in  B  (+)  partly  absorbed  and 
overgrown  by  the  new  growth.  A  new  Up  (0)  is  formed  after  a  period  of  growth 
and  this  is  again  partly  absorbed  and  overgrown  (o,  in  C).  In  C,  nine  or  ten 
successive  stages  of  growth  may  be  counted  by  the  remnants  of  the  lips.  X 1/2. 


352 


GENERAL    PRINCIPLES 


must  have  been  a  process  of  absorption  at  work  as  development 
proceeded. 

738.  Progressive  and  Regressive  Development. — Wherever 
development  is  direct  the  organization  of  the  body  is  a  con- 
tinuous progressive  process  toward  the  final  perfect  adult.  But 
when  there  is  a  change  in  the  course  of  development,  as  in  all 


FIG.  219. — Sexual  dimorphism  in  a  beetle,  Cladognathus.  The  difference 
appears  both  in  size  and  in  the  peculiar  development  of  the  mandibles  of  the 
male.  Male  on  the  left.  In  many  beetles  the  male  is  larger  than  the  female. 
Xs/6. 

cases  of  metamorphosis,  or  where  there  is  a  radical  change  in 
the  life  habits  of  the  animal,  there  is  also  a  break  in  the  con- 
tinuity of  development,  and  to  a  greater  or  lesser  degree  a  re- 
versal of  development.  This  rs  the  case,  for  example,  when  a 
free-swimming  larva  becomes  fixed  in  the  adult,  or  when  a  holo- 


REGRESSIVE   DEVELOPMENT  353 

zoic  larva  becomes  paraistic  in  the  adult.  Such  changes  in- 
volve a  loss  of  function,  or  at  least  a  change  of  function  of  some 
organs,  and  hence  a  change  in  the  organs  themselves.  This  is 
called  regressive  development.  When  the  tadpole  develops 
legs  and  lungs  and  leaves  the  water,  some  of  its  organs  have 
become  useless.  We  need  mention  only  the  gills  and  the  broad 
fish-like  tail.  These  organs,  being  now  no  longer  needed,  un- 
dergo regressive  changes,  they  are  gradually  resorbed,  dwindle 


FIG.  220. — Male  (left)  and  female  (right)  of  a  fire-fly,  Lampyris.     The  male  has 
well-developed  wings  but  the  female  is  wingless.     X2. 

and  completely  disappear.  It  must  not  be  inferred,  however, 
that  if  any  tadpole  were  kept  in  the  water  that  these  changes 
would  not  occur.  Indeed,  these  organs  have  become  useless 
before  the  frog  leaves  the  water.  The  position  might  be  assumed 
that  the  change  of  habit  occurs  because  of  the  change  in 
organization.  (See  p.  339.) 

739.  Sexual  Dimorphism. — In  some  species  the  adult  in- 
dividuals all  strictly  conform  to  one  type.  This  is  exceptional, 
however,  and  applies  only  to  hermaphrodyte  forms  like  the 
earthworms  and  some  snails.  The  vastly  more  common  con- 
dition is  a  sexual  dimorphism;  that  is,  two  types  of  individuals 
23 


354  GENERAL   PRINCIPLES 

are  regularly  developed,  male  and  female.  The  difference  be- 
tween the  two  is  often  indistinguishable  except  in  the  gonads, 
and  it  may  require  dissection  to  determine  the  sex.  In  other 
cases  the  gonads  are  visible  through  the  transparent  wall  of  the 
body,  and  a  difference  in  color  of  those  organs  often  distinguishes 
the  sexes  (jelly  fish,  some  worms,  etc.).  More  frequently  there 
are  secondary  sexual  differences,  such  as  accessory  sexual 
organs,  egg-laying  apparatus,  or  copulating  organs  or  structures 
which  are  more  remotely  or  not  at  all  connected  with  the  function 
of  reproduction.  The  female  is  very  generally  larger  than  the 
male,  a  fact  which  is  probably  to  be  connected  with  her  greater 
trophic  functions.  This  is  notably  the  case  among  insects. 
In  a  few  remarkable  instances  the  male  is  minute,  compared 
with  the  female,  and  may  even  be  attached  to  her  as  a  parasite. 
(Ex.:  Barnacles,  Sacculina,  Oedogonium.)  Among  Mammals 
the  males  often  fight  with  each  other  for  the  possession  of  the 
females,  and  this  has  resulted  in  a  greater  development  in  size 
and  strength  of  the  males.  The  male  of  the  fur  seal  is  four 
times  larger  than  the  female.  Among  Birds  the  difference  be- 
tween the  sexes  is  most  conspicuous  with  regard  to  coloration 
and  song,  in  which  the  males  usually  far  excel  the  females. 
(See  p.  423.)  Among  butterflies  there  are  often  remarkable 
differences  in  coloration  between  the  sexes,  and  in  a  number  of 
Insects  the  male  is  winged  while  the  female  is  without  wings 
(glowworm  and  Hibernia  moth).  Among  plants,  sexual 
dimorphism  is  usually  evident  only  in  the  accessory  reproductive 
organs  (flowers). 

740.  Polymorphism. — There  is  also  a  manifolding  of  form 
types  which  has  no  direct  relation  to  sex.  It  is  best  developed 
in  lower  forms,  especially  those  which  are  colonial.  Among  the 
Hydrozoa  there  may  be  as  many  as  four  or  five  types  of  indi- 
viduals. These  may  be  classed  as  the  trophic  or  feeding  polyps, 
the  budding  polyps,  the  protective  polyps,  and  the  sexual  me- 
dusae. In  the  Siphonophores  there  are,  in  addition,  the  swim- 


POLYMORPHISM 


355 


-..  b 


s  b 


r.z.J 


FIG.  221.— Diagram  of  a  Siphonophore  colony,  b,  Float;  *.&,  swimming  bell- 
m,  mouth;  w.z.,  trophic  polyp;  p.z.,  protective  zooid;  rz\  rz\  n*  reproductive 
zooids;  t,  tentacles.  (From  Galloway,  after  Lang.) 


356 


GENERAL  PRINCIPLES 


ming  bells.  The  vibraculae  and  avicularia  of  the  Bryozoa,  are 
also  modified  zooids.  In  these  cases,  which  are  characterized 
as  stock  polymorphism,  the  differentiation  of  individuals  occurs 
in  connection  with  division  of  labor,  and  in  this  special  type 
could  only  occur  in  a  colony.  An  analogous  kind  of  polymor- 
phism occurs  in  the  social  Hymenoptera,  the  bees  and  ants,  and 
the  termites  among  the  Corrodentia.  In  these  societies  there 


—  t 


—  r. 


FIG.  223. — Hydractinia,  a  polymorphic  hydroid.  C,  Ccenosarc  covering  the 
substratum;  n,  trophic  polyps;  r,  reproductive  polyps  bearing  buds  containing 
ova;  t,  tentacles.  (From  Galloway  after  Hincks.) 

are  males,  females  and  workers.  The  latter  are  undeveloped 
females.  In  some  ants  and  termites  there  is  also  a  fourth  class, 
the  soldier,  individuals  with  exceptionally  large  heads  and 
formidable  jaw.  The  interdependence  of  the  individuals  of 
these  societies  is  almost  as  great  as  that  of  polyps  in  a 
Hydroid  colony. 

741.  Alternation  of  Generations. — When  polymorphic  types 
alternate  with  each  other  in  successive  generations  we  have  the 


ALTERNATION   OF   GENERATIONS 


357 


common  phenomenon  of  alternation  of  generations.  This 
occurs  in  the  Hydroid  colony  when  the  free-swimming  sexual 
medusa  originates  by  budding  from  a  colony,  and  itself  gives 
rise  to  a  new  colony  by  a  sexual  method.  This  type  of  polymor- 
phism is  well  exemplified  by  many  of  the  trematodes  (see  p.  368), 
and  is  particularly  widespread  among  plants.  Indeed,  all  plants 
above  the  thallophytes  undergo  a  regular  alternation  of  genera- 
tions. In  the  higher  forms  it  is  rather  obscure  and  not  easily 


FIG.  223. — Polymorphism  in  Termes  lucifugus.  A,  Adult  worker;  B,  soldier. 
Both  A  and  B  are  undeveloped  males  or  females.  C,  Perfect  insect  (male  or 
female);  D,  same  after  shedding  the  wings;  E,  young  complementary  queen;  F, 
older  complementary  queen.  Enlarged.  (From  Folsom  after  Grassi  and 
Sandias.) 


described.  In  Mosses  it  is  most  conspicuous.  The  leafy 
moss  plant  develops  from  a  spore  and  is  itself  sexual  and  de- 
velops eggs  and  spermatozoids.  From  these  are  developed  the 
spore  capsule  with  its  stalk.  These  remain  connected  with  the 
sexual  plant,  but  are  themselves  the  asexual  generation  by 
which  the  spores  are  produced. 

742.  There  are  still  other  types  of  polymorphism  of  less  com- 
mon occurrence.  Seasonal  dimorphism  occurs,  for  example, 
among  some  butterflies.  In  this  the  broods  produced  at  diff- 
erent seasons  are  often  very  differently  colored,  so  that  there 
are  summer  andjall  types  or  wet'andjiry  season  types. 


358 


GENERAL  PRINCIPLES 


743.  The  polymorphism  found  in  many  flowers,  as  a  device 
for  securing  cross  fertilization,  has  already  been  described.     A 


FIG.  224. — Passage  ways  of  the  "white-ants"  in  a  post.  The  termites  avoid 
the  light,  ordinarily,  and  hence  construct  tunnels  of  mud  to  cover  their  runways. 
These  tunnels  are  often  very  extensive  and  much  labor  is  involved  in  their  con- 
struction. This  is  performed  by  the  numerous  small  workers.  Xi/2. 


very  peculiar  type  of  polymorphism  occurs  among  spiders  and 
butterflies.    A  number  of  species  of  spider  are  known  to  pro- 


HOLOPHYTIC   ORGANISMS 


359 


duce  two  kinds  of  males,  and  among  butterflies  there  occurs  a 
duplication  of  types  of  females.  In  one  case  at  least  there  are 
said  to  be  five  kinds  of  females.  (See  p.  430.) 

744.  Life  Habits  Depending  on  Food. — The  character  of 
the  food  and  the  method  by  which  it  is  obtained  exercises  a 


FIG.  225.— Seasonal  dimorphism  in  a  butterfly  (Prioneris)  from  India.     A,  Wet 
season  form;  B,  dry  season  form.     Xs/4. 

profound  influence  upon  the  organization  of  the  body.  The 
typical  plant  absorbs  C02  and  various  mineral  salts  and  through 
photosynthesis  builds  up  its  tissues.  Such  plants  are  said  to 
be  holophytic.  The  typical  animal  ingests  organic  matter  and 


360 


GENERAL   PRINCIPLES 


prepares  it  for  absorption  and  assimilation  by  digestion.     Such 
an  animal  is  said  to  be  holozoic. 

745.  Some  times  two  organisms  of  different  kinds  are  found 
living  together  by  mutual  consent,  apparently,  and  partake  of 
the  same  food.  The  sea  anemone,  on  the  shell  of  the  hermit 


FIG.  226. — Seasonal   dimorphism   in    a  European  butterfly,  Araschnia  levana. 
Both  are  females:  A,  the  winter  form;  B,  the  summer  form.     X 2. 

crab,  is  often  quoted  as  an  example  of  this  kind.  The  anemone 
secures  fragments  of  the  crab's  food,  and  the  crab  secures  some 
measure  of  protection  by  the  presence  of  the  anemone.  Such  a 
relationship  is  called  commensalism.  More  frequently  the 
needs  of  two  organisms  are  to  some  extent  complementary,  and 
one  household  may  serve  both  to  mutual  advantage.  This  is 


ANTS   AND   APHIDS 


361 


FIG.  227. — An  example  of  a  complex  interrelationship  of  organisms.  Three 
large  brown  ants  (Camponotus?)  are  guarding  a  small  colony  of  aphids  from 
which  they  obtain  honey-dew;  the  abdomens,  of  these  three  ants  being  greatly 
distended  with  what  they  obtained  in  this  way.  The  day  after  the  above  sketch 
was  made  the  large  ants  had  been  driven  away  by  a  large  band  of  small  black 
ants,  which  then  took  possession  of  the  aphid  colony.  The  aphids  in  this  case 
are  feeding  on  a  fungus  (Peridermium?)  which,  in  turn  is  parasitic  in  the  bark 
of  the  trunk  of  a  pine  tree.  Ants  are  known  to  care  for  the  eggs  of  aphids  during 
the  winter,  and  carry  the  young  to  appropriate  food  plants,  and  then  guard  the 
aphid  colony  from  the  attacks  of  other  predatory  insects.  For  this  service  the 
aphids  pay  a  tax  in  honey-dew.  The  honey-dew  is  a  clear,  sweet  fluid  secreted 
in  drops  from  the  anus  (not  from  the  tubules  on  the  dorsal  surface  of  the  abdo- 
men). The  ants  stimulate  the  aphids  by  stroking  them  with  their  antennae; 

to  this  thp.  anViirl<;  rp^nnnH  V»v  voirlintr  n   rlrrmlpf  r>f  flip 


362  GENERAL   PRINCIPLES 

symbiosis,  and,  as  examples,  we  may  refer  to  the  green  hydra, 
the  green  fresh- water  sponge  and  some  Protozoa  in  which  cells  of 
a  unicellular  alga  have  found  the  conditions  of  life  favorable 
within  the  protoplasm  of  the  animal  host.  The  CO2  eliminated 
by  the  animal  tissues  is  food  for  the  plant,  while  the  O  eliminated 
by  the  plant  cells  is  equally  useful  to  the  animal.  A  similar 
relation  exists  between  algae  and  fungi  of  many  kinds  in  the 
group  of  organisms  called  Lichens.  Here  the  algae  are  wound 
about  by  the  mycelium  of  the  fungus  so  that  they  seem  to  form 
a  single  organism.  It  has  been  found,  however,  that  they  may 
be  separated  and  grown  independently  of  each  other,  and  in 
their  structure  they  show  their  identity  or  near  relationship 
with  algae  and  fungi  which  are  found  in  nature  unconnected. 
The  mutual  advantage  here  is  probably  like  that  in  the  case  of 
hydra,  and  in  addition  the  alga  is  protected  from  the  dry  air 
by  the  dense  tissue  of  the  fungus,  and  the  fungus  possibly 
secures  soluble  food  from  the  alga. 

746.  It  is  difficult  to  judge  of  the  degree  of  helpfulness  or 
harm  which  one  organism  exercises  over  another  in  such  a 
common  household.     A  long  list  of  examples  like  the  following 
might  be  enumerated:     The  little  oyster  crab  which  is  found 
at  home  within  the  shell  of  the  oyster.     A  similar  crab  is  found 
in   the    tube    with    certain    marine    annelids    (Chaetopterus). 
Certain  ants  capture  the  cocoons  of  other  ants  and  rear  the 
young  as  slaves.     Among   ant   colonies   are   found   a   variety 
of   other  insects  living  in  more  or  less  harmony,  though  not 
always  by  the  consent  of  the  ants.     Among  the  tentacles  of 
certain  jelly   fish   (Cyanaea)   a  small  fish  is    usually    found. 
Among  higher  animals  a  companionship  between   birds  and 
mammals  is  often  observed. 

747.  Parasitism. — More  commonly  the  relationships  of  this 
sort  are  decidedly  disadvantageous  to  the  one  party.     This 
is  then  called  parasitism,  which  in  several  respects  is  one  of  the 


PARASITIC   FUNGI  363 

most  important  biological  phenomena,   and  merits   extended 
study. 

748.  The  common  mildews,  lilac  or  grape  mildew,  which 
are  seen  in  late  summer  as  a  whitish  "fur"  on  the  surface  of 
leaves,  is  due  to  a  mildew  spore  which,  blown  by  the  wind, 
falls  upon  the  leaf  and  germinates.  It  puts  out  a  slender  tube 
which  grows  through  a  stoma  into  the  mesophyll.  Here  it 


FIG.   228. — Peridermium,  a  rust  fungus  parasitic  on  pine  trees.     The  white 
ridges  are  composed  of  masses  of  spores.     X2/3. 

develops  by  absorbing  its  nourishment  from  the  mesophyll 
cells,  until  finally  it  puts  numerous  branches  out  through  the 
stomata,  and  on  each  of  these  are  borne  numerous  spores. 
The  orange-colored  or  black  specks  which  appear  later  on  the 
surface  of  the  leaf  are  spore  cases  in  which  a  second  kind  of 
spore  is  produced  from  the  same  mycelium. 

749.  The  rusts  which  occur  on  our  cereal  grasses,  wheat, 


GENERAL   PRINCIPLES 


oats,  etc.,  so  much,  have  a  complicated  life  history.  One  of 
the  most  complicated  is  that  of  the  common  wheat  rust.  In 
the  spring  the  winter  spores  (teleutospores) 
germinate,  produce  a  short  mycelium  on  which 
four  small  spores  of  another  kind  (sporidia)  are 
borne.  These  germinate  on  the  surface  of  the 
barberry  leaf,  enter  the  mesophyll  by  the 
stomata  and  develop  a  mycelium.  What  is 
called  a  cluster  cup  is  then  formed  just  under 
the  lower  epidermis.  This  is  filled  with  spores 
(aecidiospores),  and  when  the  epidermis  finally 
breaks,  the  spores  are  set  free.  These  then 
germinate  on  the  wheat  leaf  and  in  its  tissues 
a  fourth  kind  of  spore  appears  in  such  masses 
as  finally  to  burst  the  epidermis  and  produce 
long,  narrow  orange-colored  pustules  filled  with 
summer  spores  (uredospores).  These  may 
germinate  in  the  same  way  and  produce  new 
generations  of  uredospofes.  Later  in  the  season 
still  another  kind  of  spore  appears  among  the 
summer  spores,  or  in  clusters  by  itself.  These 
have  thick,  dark-colored  walls,  and  make  black 
patches  on  the  leaf.  These  are  the  winter 
spores,  teleutospores,  which  germinate  in  the 
next  spring  and  start  a  new  cycle. 

750.  The  common  cedar  apple  is  the  teleuto- 
spore-bearirig   stage   of   a   rust  which  has  its 
cluster  cup  on  the  leaves  of  the  haw. 

751.  These   parasites    are  fungi.     They   are 
typical  plant  parasites  and  often  greatly  damage 
the  host,  as,  e.  g.,  the  wheat  rust.     Less  im- 
portant, economically,    are    the   phenogamous 

parasites.     The   Indian  Pipes   are   common  flowering  plants 
growing  on  the  roots  of  other  plants.     The  parasite  has  no 


FIG.  229.— A 
fungus,  Cordy- 
ceps  ravenelii, 
parasitic  in  the 
grub  of  a  beetle, 
L  a  c  h  n  o  s  terna. 
Two  long  stro- 
mata  of  the 
fungus  are  seen 
growing  from  the 
body  of  the  grub. 
(From  Folsom 
after  Riley.) 


PARASITIC  PHENIGAMS 


365 


chlorophyll  and  the  leaves  are  scale-like.  Only  the  flowers 
are  like  those  of  normal  holoplytic  plants.  The  dodder  (love 
vine,  golden  thread)  is  a  curious  example.  When  the  seed 
germinates  on  the  ground  a  slender,  leafless  stem  grows  out. 
It  does  not  root  in  the  ground,  but  lies  flat  on  the  surface.  In 
that  way  it  continues  to  grow  at  one  end,  and  if  necessary,  at 


FIG.  230.— Dodder,  or  golden  thread  (Cuscuta).  The  weed  host  is  completely 
overspun  by  the  parasite.  The  flowers  and  seed  pods  of  the  dodder  are  seen  in 
great  numbers.  Xi/2. 

the  same  time,  absorbs  its  substance  at  the  other  end,  so  that 
it  grows  along  without  having  yet  received  any  nourishment 
except  what  was  contained  in  the  seed.  When  by  this  creep- 
ing along  the  ground  it  comes  in  contact  with  certain  green 
plants,  it  attaches  itself  to  them.  It  sends  little  root-like 


366  GENERAL  PRINCIPLES 

structures  (haustoria)  into  the  stem  of  the  host  and  from  its 
tissues  absorbs  nourishment.  Then  it  continues  to  grow, 
climbing  upon  the  host  and  winding  about  from  stem  to  branch, 
and  from  one  plant  to  another,  until  a  veritable  tangle  of  golden 
threads  is  spun  about  the  hosts.  The  leaves  of  the  dodder  are 
minute  scales.  It  has  no  chlorophyll  and  its  mode  of  nutrition 
is  wholly  parasitic.  Small  white  flowers  are  finally  produced  and 
seeds  as  in  normal  holophytes.  Our  American  "mistletoe" 
is  only  partially  parasitic. 

752.  Among  animals  we  likewise  find  parasitism  more  com- 
mon among  lower  forms.     Among  Vertebrates  only  the  round- 
mouth  eel,  the  lowest  of  fish-like  forms,  deserves  the  name 
parasite.     The  sponges  and  Echinodermes  contain  no  parasites ; 
Ccelenterates  and  Molluscs  very  few.     The  unsegmented  worms 
are  the  largest  contributors  to  the  list,  and  Insects  follow  closely. 
The  Cestodes  and  Trematodes  are  the  most  common  internal 
parasites,  and  to  the  order  Hemiptera  belong  most   of   the 
external  parasites. 

753.  The  Trematodes  are  in  some  respects  comparable  with 
the  rusts,  especially  with  regard  to  the  complicated  life  history, 
multiplicity  of  methods  of  reproduction  and  tendency  to  alter- 
nate hosts.     As  an  example  often  described  we  may  take  the 
liver  fluke. 

754.  The  adult  fluke  lives  in  the  liver  of  the  sheep  and  matures 
many  thousands  of  eggs,  which  pass  down  the  bile  ducts  into  the 
intestine,  and  thus  to  the  exterior.     From  these  eggs  hatches 
a  free-swimming  larva,  provided  the  eggs,  washed  by   the 
rain,  or  by  some  other  means,  reach  a  pond  or  stream.     The 
larva  has  a  pair  of  eye  spots,  but  is  otherwise  very  simply 
organized.     Its  further  development  depends  upon  its  coming 
in  contact  with  a  certain  species  of  fresh- water  snail.     This  pro- 
vided, it  attaches  itself  and  bores  its  way  into  the  interior 
of  the  snail,  where  it  continues  to  develop  as  a  parasite.     It 
loses  eyes  and  cilia  (sense  organs  and  locomotor  organs)  and  is 


THE   LIVER  FLUKE 


little  more  than  a  sack  (sporosac),  in  which,  by  a  process  of 

internal  budding,  a  number  of  new  individuals,  rediae,  are  pro- 

duced.    These  are  slightly  more 

highly  organized,  but  continue 

the  process  of  internal  budding 

for  several  generations.     Then 

the  rediae,  by  a  similar  process, 

develop  a  new  type  of  individ- 

uals called  cercariae.     These  are 

again    a   little    more    complex. 

They  have  two  suckers,  a  forked 

intestine  and  a  tail.     The  cer- 

cariae leave  the  snail  and  be- 

come  encysted   on    the   grass. 

If  now  they  happen  to  be  in- 

gested by  a  sheep  with  the  grass 

they  are  set  free  from  the  cyst 

in    the    stomach   of   the  iiost. 

The  parasite  may  now  be  called 

a  young  fluke,  for  if  it  succeeds 

in  finding  the  opening  of  the 

bile  duct  it  works  its  way  up 

into  the  liver  and  then  develops 

directly  into   the   fluke.      The 

mature  fluke  is  well  organized 

as   far  as  digestive  tract  and 

reproduction   Systems   are    COn- 

cerned.     The  reproductive  sys- 


_ 

FIG.  231.—  The  liver  fluke,  Fasaola 
hepatica,  showing  the  arrangement  of 


tern   especially  is  very  highly    cirrus  sac;  0,  mouth;  Ov,  oviduct,  or 

,        .  uterus;   S,   ventral  sucker;   Sg,  shell 

developed.  gland;     T,    testis;    U,   intestine;    7, 

755.  AcommonCestodeisthe    ^^  }  duct-     (From  Tyson  .after 
tapeworm.     The  one   common 

in  the  dog  may  be  taken  as  a  type.     The  eggs  originate  in  the 
intestine  of  the  dog  and  reach  the  earth  with  the  faeces.     Either 


368 


GENERAL   PRINCIPLES 


through  the  drinking  water  or  blown  in  the  dust  upon  the 
food,  these  eggs  find  their  way  into  the  stomach  of  the  rabbit. 


FIG.  232. — Diagram  of  the  life  history  of'the  liver'fluke  (Fasciola).  A,  Egg; 
B,  embryo;  C,  ciliated  larva;*/),  sporocyst;  £,  sporocyst,  later  stage;  F,  mature 
redia  containing  young  rediae  and  cercariae;  G,  cercaria;  H,  same  encysted.  /, 
young  fluke;  b,  brain;  b.p,  birth  pore;  c,  cercaria;  c.m.,  cell  masses  which  develop 
into  embryos;  e,  eye-spots;  ex.,  excretory  tubules;  g,  intestine;  m,  mouth;  ph, 
pharynx;  r,  redia;  s,  suckers;  sc,  sporocyst;  +,  stages  at  which  non-sexual  re- 
production occurs;  *,  stage  of  sexual  reproduction.  (From  Galloway  after 
Thomas,  Leuckart,  and  others.) 

Here  the  larvae  are  set  free  and  bore  their  way  into  the  tissues 
of  the  stomach,  then  get  into  the  blood  vessels  and  ultimately 


THE    TAPE    WORM 


369 


become  fixed  in  the  muscle  or  other  tissues  of  the  body.  The 
larva  develops  into  a  bladder-like,  cysticercus,  in  which  are 
formed  one  or  more  embryonic  scoleces.  In  this  condition  it 
remains  until  the  flesh  in  which  it  is  embedded  is  eaten  by  a  dog. 
Then  the  scoleces  are  set  free.  They  attach  themselves  to  the 
wall  of  the  intestine  by  means  of  the  suckers  and  hooks,  and 
then  develop  the  tapeworm  strobila.  In  this  case  the  develop- 


FIG.  233. — Diagram  of  the  tapeworm,  Taenia.  A,  Cysticercus  or  bladder- 
worm  stage.  B,  later  stage  of  same.  C,  Strobila.  The  last  proglottis  shows  the 
uterus  which  is  filled  with  embryos;  D,  one  of  the  embryos  in  the  egg  shell;  b, 
bladder;  ex,  excretory  canals;  g,  genital  pore;  h,  scolex  with  hooks  and  suckers 
(s);  «,  uterus;  z,  zone  of  strobilation.  Some  of  the  proglottides  are  numbered; 
many  are  omitted.  (From  Galloway.) 

ment  is  simpler  than  that  of  the  fluke,  and  asexual  multiplication 
may  be  confined  to  the  strobila,  as  when  the  cysticercus  de- 
velops only  one  scolex.  There  is,  however,  always  an  alter- 
nation of  hosts.  There  are  many  kinds  of  tapeworms,  each 
with  its  specific  two  hosts.  Thus,  one  tapeworm  of  the  dog 
finds  its  other  host  in  the  dog  flea.  Others  alternate  between 
cat  and  mouse,  goose  and  crayfish,  man  and  fish,  man  and  swine, 
man  and  dog,  etc.  The  tapeworm  is  a  dangerous  parasite,  not 
24 


GENERAL   PRINCIPLES 

so  much  in  the  adult  stage  as  in  the  cysticercus.  This,  embedded 
in  the  brain  or  other  organs  of  the  body,  sometimes  reaches 
enormous  size  and  destroys  the  surrounding  tissues  of  the  host. 
756.  As  internal  parasites  the  threadworms  are  very  common. 
The  famous  "horsehair  snake"  (Gordius)  is  a  parasite  in  the 


FIG.  234. — Sexually  mature  proglottis  of  Taenia.  ov,  Ovaries;  rs,  receptaculum 
seminis;  sg,  shell  gland;  t,  testis;  v,  vagina;  vd,  vas  deferens;  yg,  yolk  gland. 
Other  letters  as  in  preceding  figure.  (From  Galloway.) 

cricket  during  the  later  developmental  stages.     At  maturity  it 
is  free,  living  in  the  water. 

757.  Trichinella  spiralis  is  the  parasite  which  often  infests 
the  flesh  of  swine  and  is  frequently  transmitted  to  man  by  the 
eating  of  uncooked  pork.  The  adult  worm  is  3-4  mm.  long, 
and  bores  into  the  wall  of  the  intestine,  where  the  young  are 
produced  in  large  numbers.  The  young  are  only  .1  mm.  long, 


TRICHINE  LL  A  371 

and  are  carried  by  the  lymph  and  blood  to  the  muscle,  where 
they  remain  and  grow  to  a  length  of  i.  mm.,  and  become  en- 
closed in  a  capsule.  Unless  flesh  containing  such  encapsuled 
trichina  is  eaten  by  another  mammal,  the  development  of  the 
worm  proceeds  no  further.  When  such  flesh  is  eaten  the  cap- 
sule is  dissolved  and  the  half-grown  worm  is  set  free.  Thus  it 
gets  into  the  intestine  and  reaches  maturity  in  the  walls  of  the 
intestine.  This  parasite  may  infest  any  of  the  flesh-eating 
domestic  mammals. 

758.  There  are  a  large  number  of  intestinal  parasites  belong- 
ing to  the  round  worms  which  infest  the  intestine  of  all  the  do- 
mestic animals  and  man.     Some  of  these  (Ascaris)  reach  the 
size  of  an  earthworm  and  are  very  prolific.     The  eggs  find  their 
way  into  the  digestive  tract  of  a  host  with  the  water  and  food, 
and  their  development  takes  place  wholly  within  the  intestinal 
cavity.     These  seldom  produce  an  extreme  pathological  condi- 
tion in  the  host. 

759.  Many  smaller  threadworms  are  parasitic  in  plants. 

760.  The  parasitic  Arthropods  are  chiefly  found  in  two  or 
three  orders.     Among  the  Entomostraca  are  the  fish  lice,  chiefly 
external  parasites,  and  the  specially  notable  case  of  Sacculina. 
The  adult  Sacculina  is  found  attached  to  the  ventral  surface  of 
a  crab.     The  portion  of  it,  which  is  visible  externally,  is  little 
more  than  a  large  sack  containing  an  elaborately  developed 
reproductive  system.     The  sack  is  attached  to  the  host  by  a 
process  of  its  body,  which  penetrates  the  tissues  of  the  host,  and 
then  branches  and  penetrates  in  all  directions,  like  the  root 
system  of  a  plant.     By  this  organ  it  absorbs  nourishment  from 
the  host.     It  has  no  digestive  system.     The  young  of  this  strange 
organism  are  free-swimming  nauplii  with  eyes  and  appendages 
of  the  typical  nauplius.     But  when  the  larvae  attach  them- 
selves to  a  host,  the  appendages  and  eyes  undergo  degenera- 
tion until  there  remains  only  the  organism  as  described. 

761.  Among  Insects,  the  parasites  are  found  chiefly  among 


372 


GENERAL   PRINCIPLES 


the  Diptera  and  Hemiptera.  The  larvae  of  the  Diptera  are  often 
parasites.  The  botfly,  Hypoderma,  develops  under  the  skin  of 
cattle.  The  botfly  of  the  horse  (Gastrophilus)  deposits  its  eggs 
about  the  shoulders  and  head  of  the  horse.  The  horse  gnaws 
them  off  or  they  fall  into  his  food,  and  thus  get  into  the  stomach, 
where  the  larvae  remain  attached  to  the  wall  of  the  stomach. 
762.  The  botfly  larvae  of  Cephalomyia  ovis  in  the  frontal 
sinuses  of  sheep  produce  blind-staggers.  The  larvae  of  the 


FIG.  235. — Ichneumon  fly,  Thalessa  lunator,  depositing  eggs  in  the  burrow  of 
the  wood-boring  Tremex  upon  whose  larvae  the  larvae  of  the  Thalessa  feed. 

ichneumon  fly,  a  Hymenopter,  are  -parasitic  in  the  cater- 
pillars of  various  butterflies.  The  adults  of  a  large  number  of 
flies  are  temporarily  external  parasites,  as  are  also  many  mos- 
quitoes. The  flea  is  also  closely  related  to  the  flies,  and  its 
wingless  condition  is  probably  the  result  of  degeneration 
through  parasitism. 

763.  The  Hemiptera,  or  bugs,  are,  as  a  group,  parasitic. 
They  are  often  evil  smelling  because  of  the  secretion  of  a  peculiar 
gland.  The  bed  bug  and  squash-bugs  exemplify  this  point  well. 
The  plant  lice,  scale  insects,  the  water  striders,  water  boatmen 


PHYLLOXERA 


373 


and  electric  light  bugs  (the  last  three  are  aquatic  bugs),  the 
cicadas  (harvest  fly  and  seventeen-year-locust),  and  chinch  bugs, 
are  all  familiar  parasites,  largely  on  plant  hosts.  The  Phyllox- 
era is  parasitic  on  the  grape,  and  merits  detailed  description. 
In  the  spring  the  first  generation  of  young  hatch  from  eggs  which 
were  deposited  the  preceding  fall  under  the  bark  of  the  vine. 
This  generation  is  wingless  and  reproduces  parthenogenet- 
ically.  Successive  generations  of  similar  individuals  follow. 


FIG.  236. — A  tomato  worm  covered  with  the  cocoons  of  its  parasite,  Apanteles, 
which  is  also  a  Hymenopter.     (From  Folsom.) 

These  cause  the  galls  on  the  leaves  and  the  nodules  on  the  roots, 
for  they  also  attack  the  roots  underground.  In  late  summer 
another  type  appears.  These  are  winged  and  serve  to  scatter 
the  species.  They  lay  two  kinds  of  parthenogetic  eggs  on  the 
underside  of  the  leaves.  From  the  larger  eggs  there  develop 
females,  and  from  the  smaller  ones  males.  These  are  both 
destitute  of  digestive  tract.  The  females,  after  fertilization, 
deposit  a  single  egg  under  the  bark  of  the  vine.  These  eggs  re- 
main over  winter  and  hatch  the  first  generation  in  the  spring. 


374 


GENERAL   PRINCIPLES 


764.  The  galls  so  often  seen  on  oak  leaves  and  twigs,  and  also 
on  many  ather  plants,  are  abnormal  developments  of  the  plant 
tissue  due  to  a  stimulation  produced  by  insect  parasites.  The 
female  Cynips,  a  wasp-like  insect,  deposits  her  eggs  in  the  tissues 
of  the  plant,  and  during  the  development  of  the  young  the  tissues 


FIG.  237. — Dog  flea,  Ctenocephalus  canis.     A,  Larva;  B,  adult, 
after  Kunckel  d'Herculais.) 


(From  Folsom 


B  A 

FIG.  238. — Oak  galls  (.4)  made  by  the  gall  wasp,  Holcapsis  globulus  (B). 
Ay  natural  size;  B,  magnified  X  2.     (From  Folsom.) 

are  irritated  in  such  a  way  as  to  cause  the  abnormal  develop- 
ment of  the  surrounding  tissues.  The  gall  forms  a  shelter  for 
the  young  brood  and  the  juices  of  the  plant  provide  food. 

765.  Protozoa    As    Parasites. — Many    species    of    amoeba 
(Entamceba)  are  found,  as  parasites,  in  the  digestive  tract  and 


TRYPANO  SOMES  375 

in  other  organs  of  the  body.  They  have  been  found  in  \many 
mammals,  birds,  frogs,  and  insects.  Some  of  these  scarcely  de- 
serve the  name  parasite,  since  their  presence  in  the  digestive 
tract  seems  to  cause  the  host  no  inconvenience.  To  this  class 
belongs  Entamceba  coli,  which  is  found  in  the  human  intestine 
in  a  large  percent,  of  normal  individuals.  Entamceba  histoly- 
tica,  however,  penetrates  the  wall  of  the  intestine  and  causes 
the  disintegration  of  the  tissues,  or  ulceration.  This  is  the 
cause  of  tropical  dysentery,  a  serious  and  often  fatal  disease, 
which  is  quite  common  among  the  people  of  tropical  countries. 
766.  Among  the  Flagellates  the  Trypanosomes  are  the  most 
important  group  of  parasites.  They  find  their  hosts  among  all 


FIG.  239. — A  Trypanosome.    /,  Flagellum;  m,  undulating  membrane;  n,  nucleus. 
(From  Marshall  after  Doflein.) 

the  classes  of  Vertebrates,  as  well  as  some  invertebrates,  but 
the  Mammals  are  most  seriously  affected.  The  parasite  is 
usually  found  in  the  blood  and  causes  intermittent  fever,  swelling 
of  the  spleen  and  lymph  glands,  anaemia,  eruptions  of  the  skin 
and  disorders  of  the  nervous  system.  Trypanosoma  gambiense 
is  the  cause  of  the  terrible  "sleeping  sickness"  of  South  Africa. 
It  is  apparently  the  toxic  effect  of  the  parasite  on  the  nervous 
system  that  produces  the  later  symptoms  of  the  disease,  a 
lethargic  condition  which  slowly  leads  to  a  continuous  sleeping 
and  finally  ends  in  death.  In  large  parts  of  South  Africa  the 


376  GENERAL   PRINCIPLES 

cattle,  horses,  and  in  fact,  all  the  domestic  mammals,  as  well 
as  wild  mammals,  are  affected  by  a  disease  known  as  Tsetse 
fever.  It  is  fatal  to  such  a  degree  that  "  large  areas  are  closed 
to  colonization"  where  the  disease  is  endemic.  Trypanosoma 
Brucei  is  the  cause  of  the  fever,  but  Tsetse  is  the  name  of  a  fly. 
The  natives  have  long  known  that  the  fever  only  occurs  in 
districts  in  which  the  Tsetse  fly  is  found,  and  there  is  now  no 
doubt  that  this  fly,  in  stinging  affected  cattle  becomes  itself 
infected  and  then  carries  the  germs  to  uninfected  cattle.  There 
are  several  species  of  Tsetse  fly  (Glossinia),  and  of  these,  prob- 
ably more  than  one  is  responsible  for  the  spread  of  Tsetse  fever. 
The  sleeping  sickness  is  also  carried  by  Tsetse  flies. 

767.  The   domestic    animals    of    South   America,    southern 
Europe  and  northern  Africa,  and  the  countries  bordering  on 
the  Indian  Ocean,  are  also  affected  by  different  types  of  Trypano- 
some  diseases.     In  these  cases   other  flies  and  mosquitos  are 
the  principal  agencies  of  infection,  but  lice  and  fleas  may  per- 
form the  same  office.     The  Trypanosomes  of  fishes  are  carried 
by  leeches. 

768.  Of  more  direct  interest  to  us  is  the  parasite  of  malarial 
fever.     There  are  at  least  three  varieties  of  this,  producing  the 
" tropical, "  the  " tertian, "  and  the  "quartan"  fevers,  respec- 
tively.    At  a  certain  stage  there  are  found  numerous  minute 
bodies  floating  in  the  blood  plasma  of  the  host.     These  are 
the  "spore"  stage  of  a  Sporozoan,  Plasmodium.     They  are 
vastly  smaller  than  a  red  blood  corpuscle  and  are  capable  of 
amoeboid  motion.     They  attach  themselves  to  a  red  corpuscle 
and  work  their  way  into  it.     Here  they  grow  at  the  expense  of 
the  blood  corpuscle,  and  at  the  end  of  48  hours,  in  the  case  of 
the  tertian  parasite,  they  divide  into  a  number  of  "spores." 
Hereupon  the  corpuscle  goes  to  pieces  and  the  spores  are  again 
floating  in  the  plasma.     These  spores  repeat  the  cycle  just  de- 
scribed and  thus  a  new  generation  of  "spores"  is  produced  on 
each  alternate  day.     This  process  may  continue  indefinitely, 


PLASMODIUM 


377 


FIG.  240. — Life  history  of  malarial  parasite,  Plasmodium.  i,  Sporozoite 
introduced  into  human  blood  by  bite  of  mosquito;  2,  same  a  little  later;  3  and 
4,  same  growing  in  a  red  blood-corpuscle;  5,  same  dividing;  6,  blood-corpuscle 
disintegrated  and  setting  free  the  spores;  7,  8  and  9,  a  spore  developing  into  a 
female  gamete;  70,  8a,  ga  and  gb,  a  spore  developing  into  a  number  of  male 
gametes;  10,  union  of  male  and  female  gametes  (fertilization);  n,  motile  zygote; 
12,  zygote  embedded  in  the  wall  of  the  stomach  of  the  mosquito;  13  to  1 6,  stages 
in  the  development  of  sporozoites  in  the  sporocyst;  17,  sporozoites  in  the  salivary 
gland  of  the  mosquito.  Stages  from  i  to  8  in  the  human  blood.  Stages  8  to  17 
in  the  mosquito.  (From  Folsom  after  Grassi  and  Leuckart.) 


378  GENERAL  PRINCIPLES 

but  at  intervals  another  type  of  development  occurs  side  by 
side  with  the  spores.  In  this  case  the  enlarged  parasitic  cell  in 
the  blood  corpuscle  does  not  divide  into  a  number  of  spores,  but 
becomes  much  elongated  and  cresent-shaped.  Now,  however, 
for  further  development  a  change  of  host  is  necessary  and  this 
host  must  be  one  of  a  few  species  of  mosquitos  (Anopheles). 
In  the  stomach  of  the  mosquito  the  crescents  just  described  be- 
come differentiated  into  two  classes.  In  the  one  class  the 
crescents  become  rounded  and  motionless,  while  in  the  other 
division  occurs  and  a  number  of  very  long  and  slender  motile 
bodies  are  formed.  These  are  female  and  male  gametes  re- 
spectively, and  a  fusion  takes  place  between  them  as  in  fertili- 
zation. The  zygote  now  becomes  motile.  It  works  its  way 
into  the  wall  of  the  digestive  tract  where  it  remains  for  about 
eight  days  while  undergoing  further  development.  This  is 
called  the  sporocyst  stage.  During  this  period  it  grows  to  an 
enormous  size,  it  first  divides  into  a  number  of  cells  and  these 
then  each  develop  a  vast  number  of  sporozoites,  very  long  and 
very  slender  spindle-shaped  motile  spores.  By  the  bursting 
of  the  sporocyst  the  spores  escape  into  the  body  cavity  and  thus 
gain  access  to  the  salivary  glands.  For  some  reason  they  work 
their  way  into  the  salivary  glands  and  their  ducts,  and  hence, 
when  the  mosquito  next  punctures  the  skin  of  an  uninfected 
person  some  of  the  sporozoites  are  carried  with  the  saliva  into 
the  wound,  and  the  victim  is  thus  inoculated  with  malarial 
virus.  The  sporozoites  are  merely  another  type  of  spore. 
They  attack  the  red  blood  corpuscles  in  the  same  way  as  in  the 
stage  with  which  we  began. 

769.  We  see  that  the  malarial  parasite  completes  its  life 
history  only  by  transferring  from  man  to  mosquito.     Both 
hosts  are  necessary.     This  is  probably  also  true  of  the  try- 
panosomes,  as  is  indicated  by  the  more  recent  investigations. 

770.  The  fever  days  of  malaria  are  the  days  in  which  the 
new  generation  of  spores  are  set  free  by  the  disintegration  of  the 


BACTERIA  379 

blood  corpuscles.  This  occurs  every  other  day  in  tertian  fever, 
and  on  every  third  day  in  quartan  fever.  A  double  inoculation 
may  result  in  a  more  complicated  succession  of  fever  days. 

771.  The  Apes,  Bats  and  Birds  are  also  subject  to  Plas- 
modium  parasites.     Texas  cattle  fever  is  caused  by  a    elated 
Sporozoan  (Babesia),  which  is  transmitted  by  the  cattle  tick. 

772.  Bacteria  as  Parasites. — Most  infectious  diseases  are 
caused  by  bacteria.     This  has  been  definitely  established  for 
many  diseases,  but  the  difficulties  in  the  way  of  determining  a 
causal  relationship  between  such  minute  organisms  and  the 
diseases  with  which  they  are  supposed  to  be  associated  are 
often  very  great,  and  in  a  number  of  cases  the  organism  has 
not  yet  been  identified,  though  the  disease  is  almost  certainly 
known  to  be  bacterial.     The  part  of  the  body  infested  by  the 
parasites  varies  with  the  species;  sometimes  it  is  the  mucous  sur- 
faces of  the  digestive  tract  or  the  respiratory  passages,  some- 
times the  tissues  of  certain  organs,  sometimes  the  blood  vessels 
and  lymph  spaces  of  certain  organs  or  even  of  the  entire  body. 
The  mode  of  infection  also  varies  with  the  disease.     Some- 
times the  germs  find  their  way  to  the  host  with  the  food,  water 
or  air  of  respiration,  but  they  may  also  enter  the  body  through 
the  skin.     The  latter  is  not  likely  to  occur  except  when  the  skin 
is  broken. 

773.  The  effect  of  the  parasite  on  the  host  is  sometimes 
limited  to  a  disorganization  of  the  tissues  of  a  limited  region. 
The  result  of  this  may  not  be  serious,  but  if  the  destruction 
of  the  tissues  goes  far  in  a  vital  organ  the  function  of  the  organ 
may  be  seriously  impaired  and  result  in  the  death  of  the  host. 
In  other  cases  no  anatomical  change  can  be  observed  in  the 
tissues,  and  yet  the  function  of  some  organs  may  be  disturbed 
and  consequently  the  life  of  the  host  threatened. 

774.  Bacteria  vary  like  other  classes  of  parasites  with  regard 
to  the  range  of  hosts  in  which  they  may  be  found.     But  this 
question  has  been  studied  more  particularly  from  the  point  of 


380  GENERAL   PRINCIPLES 

view  of  the  host  and  has  led  to  several  very  important  general 
conclusions  concerning  the  degree  of  susceptibility  of  species  or 
of  individuals  to  given  bacterial  diseases.  From  what  is  said 
concerning  the  physiological  processes  of  bacteria  in  the  Ap- 
pendix to  Part  I,  it  may  be  suspected  that  the  effect  produced 
on  the  host  by  the  bacterial  parasite  is  due  to  substances 
secreted  by  the  bacteria.  As  a  matter  of  fact,  it  is  found  that 
if  an  extract  from  the  bacterial  cultures  containing  no  living 
cells  is  introduced  into  the  body  of  the  host,  the  symptoms 
peculiar  to  the  corresponding  disease  are  produced.  The  bac- 
terial products  are  similar  to  poisons  in  their  effects,  and  are 
called  toxins. 

775.  Immunity. — When  an  individual  or  a  species  is  not 
susceptible  to  the  attack  of  an  infectious  disease,  it  is  said  to  be 
immune.     The  immunity  may  be  a  native  character  of  the 
animal,  it  may  be  acquired  during  the  life  time  of  the  individ- 
ual through  natural  causes,  or  it  may  be  induced  artificially. 
These  types  are,  therefore,  designated  natural  immunity  and 
acquired  immunity,  respectively,  and  of  the  latter  there  are 
two  types,  active  and  passive. 

776.  Natural  immunity  is  due  primarily  to  three  kinds  of 
defense,  which  the  organism  employs  to  defend  itself  against 
bacteria  which  have  succeeded  in  entering  the  body,     (i)  Any 
foreign  particles  introduced  into  the  tissues  and  causing  irri- 
tation  are   attacked   by   the   white  blood   corpuscles.     These 
cells  are  capable  of  independent  locomotion  through  amoeboid 
movements.     They  escape  from  the  blood  vessels  by  pene- 
trating the  walls  and  move  about  in  the  lymph  spaces  in  the 
tissues.     They  collect  about  foreign  matter  and  engulf  and 
digest  particles  as  would  an  amoeba.     This  process  of  phagocy- 
tosis is  regarded  as  of  great  importance  in  keeping  the  body 
free   of   bacterial   invasions.     (2)  The   blood   of   a   naturally 
immune  animal  contains  a  substance,  alexin,  which  causes  the 
death  of  the  bacteria.     This  substance  is  probably  formed  by 


IMMUNITY  381 

the  cells  of  the  various  tissues.  (3)  It  is  generally  true  that 
individuals  are  not  equally  susceptible  to  tht  same  poisons  and 
natural  immunity  rests,  in  part,  on  this  fact.  The  immune 
individual  is  not  affected  by  the  bacterial  products  which  act  as 
toxins  in  other  individuals.  This  is  accounted  for  by  the  pres- 
ence in  the  blood  of  the  immune  individuals  of  a  substance, 
which  neutralizes  the  toxins,  and  is,  therefore,  called  antitoxin. 

777.  The  animal  organism  is  not  passive  to  the  attacks  of 
bacteria.     If  the  attack  is  not  too  sudden  and  violent   the 
tissues  respond  by  producing  antitoxin,  which  neutralizes  the 
effect  of  the  toxin,  and  a  lysin  (alexin),  which  causes  the  dis- 
solution of  the  bacteria.     Another  substance  is  also  formed 
which  causes  the  bacteria  to  adhere  in  clumps.     This  is  called 
agglutinin.     These  substances  (anti-bodies)  produced  by  the 
tissues  in  response  to  the  bacterial  stimulus,  may  continue  to  be 
formed  long  after  the  exciting  cause  has  disappeared  and  the 
body  is  therefore  immune  to  a  second  attack.     This  is  known 
as  acquired  immunity. 

778.  Bacteria  are  extremely   variable.     This   is    especially 
evident  in  the  degree  of  virulence  of  different  strains  of  what  is 
apparently  the  same  species.     Immunity  acquired  from  the 
attack  of  a  mild  form  is  also  generally  efficient  against  the  more 
virulent  types.     This  principle  is  employed  to  produce  immunity 
by  intentionally  infecting  or  inoculating  an  individual  with  a 
mild  type  and  thereby  securing  protection  against  more  danger- 
ous forms.     Immunity  secured  in  this  way  is  active  acquired 
immunity. 

779.  Passive  immunity  is   secured  by  injecting   into    the 
animal  a   blood   serum    obtained   from  an  immune  animal. 
This  serum-therapy  is  effective  at  the  time,  but  the  anti- 
bodies soon  disappear   from    the   blood   and,  since  the  tis- 
sues have  not  been  stimulated    to    the   formation   of    anti- 
bodies, the  immunity  is  lost. 

780.  The  various  types  of  immunity  may  be  illustrated  by 


382  GENERAL  PRINCIPLES 

a  few  familiar  forms.  Man  is  naturally  immune  to  fowl  cholera, 
though  sometimes  attacked  by  cattle  fever,  anthrax.  The  fowl 
is  immune  to  rabies,  to  which  both  man  and  the  dog  are  subject. 
The  dog  is  immune  to. anthrax.  By  an  attack  of  measles  or 
smallpox  man  acquires  immunity  against  subsequent  attacks. 
By  vaccinating  man  with  the  virus  of  cowpox,  a  mild  disease 
is  produced,  which  renders  the  individual  actively  immune 
against  smallpox.  Passive  immunity  against  diphtheria  is  se- 
cured by  the  injection  of  an  antitoxin  serum  taken  from  an 
actively  immune  horse. 

EVOLUTION 

781.  Species. — The  word  species  is  one  of  the  most  important 
and  most  frequently  employed  of  all  biological  terms,  and  yet 
it  is  impossible  of  exact  definition.  A  species  is  a  kind  of  a 
plant  or  animal,  using  the  word  kind  in  its  most  common  sense. 
Thus  the  sweet-gum  (Liquidambar  Styraciflua),  the  persimmon 
(Diospyros  Virginiana),  and  the  tulip  tree  (Liriodendron  tulipi- 
fera),  are  clearly  defined  species  as  are  also,  among  animals,  the 
turkey  vulture,  or  buzzard  (Cathartes  aura)  and  the  robin 
(Turdus  migratorius) .  Any  given  example  may  immediately 
be  recognized  as  sweet-gum  or  not-sweet-gum,  as  robin  or  not- 
robin.  In  other  cases,  however,  difficulties  may  arise.  Thus, 
among  the  oaks  we  have  the  willow  oak,  the  blackjack,  the  white 
oak  and  the  Spanish  oak,  all  of  which  are  distinct  species  and 
readily  distinguishable.  But  it  frequently  occurs  that  the 
flowers  of  one  species  are  pollinated  by  those  of  another  and  the 
resulting  offspring  is  called  a  hybrid.  It  resembles  both  parent 
species  to  some  extent,  but  belongs  to  neither.  A  much  more 
important  difficulty  arises  from  individual  variation,  for  the 
individuals  of  a  species  are  never  exactly  alike.  Even  the  in- 
dividuals sprung  from  the  same  parents  may  vary  greatly 
among  themselves.  Allowance  must,  therefore,  be  made  for 


VARIATION  383 

individual  variation  within  the  species.  Sometimes  certain 
types  of  variation  occur  so  constantly  that  the  species  may  be 
subdivided  into  varieties.  This  is  especially  true  where  a 
species  is  widely  distributed  and  thus  lives  under  different 
conditions  in  the  various  districts  it  inhabits.  Where  there 
is  variation  corresponding  to  geographical  distribution  it  is 
called  a  geographical  variety.  A  good  example  of  this  is 
Lycaena  pseudargiolus,  the  common  small  blue  butterfly,,  which 
ranges  from  New  England  to  Arizona.  The  New  England, 


FIG.  241. — Two  flower  heads  of  Gaillardia.  The  head  on  the  right  is  the  nor- 
mal type,  with  ray  flowers  ligulate.  The  head  on  the  left  is  a  variation  (sport) 
which  frequently  occurs.  The  ray  flowers  in  this  are  tubular  and  often  quite 
regular.  X2/3. 

Middle,  Southern  and  Southwestern  States  varieties  of  this 
butterfly  differ  so  much  that  they  might  well  be  classed  as  four 
distinct  species,  were  it  not  for  the  intergrading  forms  found 
in  the  transition  regions.  Nor  is  this  an  isolated  example. 
Extended  study  of  a  species  almost  invariably  widens  the  range 
of  its  recognized  variability,  and  sharply  defined  species  are  the 
exception  rather  than  the  rule. 

782.  Opposed  to  the  tendency  to  vary  is  a  tendency  for  the 
species  to  maintain  its  character.  This  is  evident  in  the  re- 
semblance of  the  offspring  to  the  parents.  Although  the  in- 


GENERAL   PRINCIPLES 


(Third  filial  generation). 

FIG.  242. — Mendelian  inheritance  in  the  four-o'clock.  If  red  (a)  and  white 
(b)  forms  are  crossed  the  offspring  are  all  pink  (c).  These  interbred  yield  1/4 
red  (d),  2/4  pink  (e  and/),  and  1/4  white  (g).  The  lower  part  of  the  figure 
shows  the  condition  of  the  zygote  and  the  gametes  in  the  ancestral  and  first, 
second,  and  third  filial  generations.  (From  Davenport  after  Haecker.) 


MENDEL'S  LAWS  OF  HERIDITY  385 

dividuals  of  the  same  brood  differ  among  themselves  and  from 
their  parents  they  still  resemble  their  parents,  and  hence  each 
other,  more  than  they  do  distant  relatives  or  unrelated  members 
of  the  same  species.  That  is  to  say,  the  peculiarities  of  the 
parents  tend  to  reappear  in  the  offspring.  This,  in  some  cases, 
has  been  found  to  be  controlled  by  very  simple  laws,  known  as 
Mendel's  Laws  of  Heredity.  If  the  two  parents  differ  with 
regard  to  a  certain  character  the  offspring  of  the  first  generation 
will  inherit  equally  from  both  parents.  In  the  second  genera- 
tion one-fourth  will  have  the  original  paternal  character  only, 
one-fourth  will  have  the  original  maternal  character  only,  and 
the  remaining  two-fourths  will  still  be  mixed  like  the  first  gen- 
eration. In  the  third  generation  the  mixed  two-fourths  of  the 
preceding  generation  will  be  divided  into  a  fourth  pure  paternal, 
a  fourth  pure  maternal,  and  two-fourths  mixed.  In  the  fourth, 
fifth,  etc.,  generations  this  process  will  continue.  In  this  it 
must  be  recognized  that  a  character  may  be  present,  though  not 
evident.  Such  a  character  is  said  to  be  recessive  while  the  op- 
posed character,  which  is  evident,  is  said  to  be  dominant.  To 
illustrate  we  may  take  a  simple  case.  If  the  red  and  white 
varieties  of  Mirabilis  Jalapa  (four  o'clock)  are  crossed,  the  off- 
spring in  the  first  generation  are  all  pink.  The  second  genera- 
tion (secured  by  close  fertilization  of  the  pink  generation), 
however,  consists  of  three  kinds,  viz.:  One-fourth  white,  two- 
fourths  pink,  and  one-fourth  red.  The  white  and  red  forms 
are  said  to  be  pure  because  they  continue  to  produce  only  white 
and  red,  respectively,  in  succeeding  generations  if  close  fertil- 
ized. The  other  two-fourths  of  pink  flowers  in  the  next 
generation  again  break  up  into  white,  pink,  and  red  forms  in 
the  proportion  of  1:2:1,  as  before,  and  thus  the  pink  or  mixed 
forms  continue  in  each  generation  to  separate  into  the  three 
kinds. 

783.  When   the   two  parental   characters  are  not  equally 
potent,  i.  e.,  when  one  is  dominant,  the  other  recessive,  the 
25 


386  GENERAL  PRINCIPLES 

results  are  as  follows:  We  will  take  as  an  example  the  pure 
white  and  common  gray  mice.  The  result  of  this  cross  is 
gray  mice  like  the  gray  parent,  not  a  lighter  gray,  as  one  might 
expect,  following  the  case  of  Mirabilis  Jalapa.  In  the  second 
generation  there  are  a  fourth  white,  which  are  pure,  and  three- 
fourths  are  gray.  The  gray  are  in  reality  of  two  kinds,  though 
this  becomes  evident  only  in  the  course  of  succeeding  genera- 
tions, when  it  develops  that  one-third  of  the  three-fourths 
continue  to  breed  only  gray,  while  the  other  two-thirds  yield 
one-fourth  white  in  the  succeeding  generation  and  are  thus 
seen  to  have  been  mixed.  The  second  generation  of  offspring 
may,  therefore,  be  described  as  one-fourth  white,  two-fourths 
mixed  with  gray  dominant,  and  one-fourth  pure  gray.  The 
dominant  grays  and  pure  grays  can  only  be  distinguished  by 
the  character  of  their  offspring. 

784.  Not  every  character  is  controlled  in  this  simple  way, 
for  it  is  readily  conceivable  that  a  given  character  may  be  due 
to  the  combined  operation  of  several  factors,  each  of  which  may 
be  separately  heretible. 

785.  Physical  Basis  of  Heredity. — The  phenomena  of  he- 
redity correspond  in  a  remarkable  way  with  those  of  matura- 
tion, which  makes  plausible  the  theory  that  the  chromosomes 
are  the  bearers  of  the  hereditary  traits  of  the  organism,  and 
that  during  maturation  the  readjustment  of  chromatin  deter- 
mines the  ancestral  characters  which  are  to  be  handed  on  to  the 
next  generation.     In  the  process  of  fertilization  the  germ  cell 
is  provided  with  equal  parts  of  maternal  and  paternal  chroma- 
tin,  and  this  condition  is  maintained  during  the  subsequent 
stages  of  development  in  all  the  cells  of  the  body.    Let  the 
condition  of  the  chromatin  with  regard  to  any  hybrid  character 
be  represented  by  (P.M.).     Then  when  the  first  maturation 
division  occurs  in  the  primary  oocyte  of  the  hybrid  (F.)  the 

/T»  •»*•  \    i  j-  -j    •  j.      { (P.M.)  =  ist  polar  body 

(P.M.)  elements  divide  into  ?  ;*    /; 

(  (P.M.)  =  secondary  oocyte. 


THE    CHROMOSOMES   AND   HEREDITY  387 

But   when   the   reduction   division   occurs   the   paternal   and 
maternal  elements  are  separated  and  the  result  is  ........... 

.........  either  ....................  or 

J    (P.M.)    j  =    ist  p.  'b.=  f    (P.M.)    1 
2dp.b.=  {  (P.)  (M.)  j  =mature  eggs=  {  (P.)(M.)  j  =2d  p.  b. 
If  the  first  polar  body  divides  the  result  is 

^  \     /i/\  r  in.  which  either  one  of 

(P.)    (M.)  j 

the  four  cells  may  represent  the  ripe  egg  and  the  other 
three  the  three  polar  bodies.  The  ova  are,  therefore,  either 
(P)aternal  or  (M)aternal  in  regard  to  the  hybrid  character,  and 
the  two  kinds  are  probably  equally  numerous. 

786.  In  the  maturation  of  the  sperm  the  process  is  similar. 
The  primary  spermatocyte  (P'.M'.)  divides  into  two  secondary 
spermatocytes, 

(P'  M'  ) 

,     '      /'  which  then  divide   into 


. 
/p/\   /;*f/  \  f°ur    spermatids.     Two    of    these    are    (P')ater- 

nal  and  two  are  (M')aternal,  so  that  the  number  of  (P'.)  and 
(M7.)  sperm  is  also  equal,  as  in  the  case  of  the  egg. 

787.  By   close   fertilizing,    the   gametes   of   the   FI    hybrid 
generation  may  combine  as  follows: 

(P.P'.),  (P.M'.),  (P'.M.),  (M'.M.).  This  results  in  one-fourth 
pure  paternal,  two-fourths  hybrid  and  one-fourth  pure  maternal 
individuals  in  the  F2  generation. 

788.  Number  of  Species.  —  In  spite  of  the  fact  that  it  is 
often  extremely  difficult  and  even  impossible  to  definitely  cir- 
cumscribe a  species,  yet  for  convenience  of  reference  animals 
and  plants  must  be  divided  into  convenient  groups,  named  and 
classified.     And  much  time  has  been  spent  in  hunting  out  and 
describing  species.     About  520,000  animal  species  have  thus 
been  listed  and  over  200,000  plants.     Besides  these  there  are 
many  species  which  have  become  extinct  and  are  now  only 


388  GENERAL  PRINCIPLES 

represented  by  fossil  remains.  Of  these  there  are  about  60,000 
known.  In  addition  to  these  there  are  many  species  living 
which  have  not  been  observed  by  the  recording  biologist,  and 
hence  do  not  appear  in  the  count,  and  there  have  probably 
been  many,  many  more  which  became  extinct  without  leaving 
any  trace  or  the  remains  of  which  have  not  yet  been  found.  It 
is,  therefore,  not  probable  that  any  one  familiar  with  the  facts 
would  regard  a  million  a  high  estimate  for  the  number  of  species 
which  have  been  or  are  now  living  on  the  earth. 

789.  Origin    of    Species. — The    question    naturally    arises, 
Whence  came  they  all?     It  is  a  question  which  has  always 
occupied  thinking  men,  and  concerning  which  there  has  been 
much  difference  of  opinion.     To-day  biologists  generally,  if 
not  all,  are  of  the  opinion  that  species  are  plastic,  as  it  were,  and 
continually  undergoing  modification,  so  that  they  are  not  to-day 
what  they  were  or  what  they  will  be,  and  further  that  two  sec- 
tions of  a  species  may  become  modified  in  different  directions 
and  thus  come  to  differ  even  to  the  extent  of  specific  distinction. 
In  this  way  there  would  arise  two  species  where  there  had  been 
but  one.     This  is  known  as  the  theory  of  the  origin  of  species 
by  descent  with  modification.     In  connection  with  this  theory 
there  are  two  questions  which  should  be  clearly  distinguished. 
The  first  one- is  as  to  fact,  the  other  as  to  method:  (i)  What 
is  the  evidence  that  species  originate  by  descent  with  modi- 
fication?    (2)  If  a  fact,  how  does  it  come  about?     In  the  fol- 
lowing pages  we  will  consider  the  evidence  upon  which  the 
theory  of  descent  rests,  and  in  that  connection  take  up  a  number 
of  important  biological  phenomena  which  have  not  yet  been 
discussed. 

790.  The  Taxonomic  Series. — Long  ago,  students  of  natural 
history  were  struck  by  the  fact  that  animals  could  be  arranged 
in  a  series  in  the  order  of  their  various  degrees  of  organization; 
with  the  simplest  at  one  end,  the  most  complex  at  the  other,  and 
the  interval  between  more  or  less  completely  filled  by  forms  of 


THE    TAXONOMIC   SERIES  389 

intermediate  grade.  This  series  is  fairly  well  represented  by 
almost  any  scheme  of  classification  adopted  by  systematists 
to-day.  Great  similarity  in  form  and  structure  naturally 
suggests  a  blood  relationship  and  a  common  origin.  But  the 
taxonomic  series  represents  a  continuous  chain  of  such  relations, 
and  hence  leads  to  the  conclusion  that  the  entire  animal  king- 
dom had  a  common  origin,  and  that  by  a  process  of  evolution 
the  higher  forms  have  developed  from  lower  forms  as  the  com- 
plex adult  develops  from  the  simple  egg. 

791.  The    taxonomic    series    is,    however,    not    adequately 
represented  by  a  line  and  it  is  now  more  frequently  compared 
to  a  tree.     At  its  base  this  genealogical  tree  divides  into  two 
trunks,  one  representing  the  vegetable  kingdom,  the  other  the 
animal.     The  base  from  which  they  both  spring  represents  the 
many  unicellular  forms  which  have  both  vegetable  and  animal 
characteristics.     Then  up  along  the  animal  trunk  come  the  less 
differentiated  forms,  like  some  Ccelenterates,  Annelids,  Peri- 
patus,  Branchiostoma,  etc.     The  branches  represent  the  highly 
differentiated  forms  and  spring  from  the  main  stem  at  various 
points;  the  Sponges  below,  higher,  the  Echinoderms,  Arthro- 
pods, Molluscs,  etc.,  while  several  large  branches  at  the    top 
represent  bony  Fishes,  Reptiles,  Birds  and  Mammals. 

792.  With  the  advancement  of  the  study  of  anatomy  much 
information  has  been  obtained  which  throws  light  on  this  ques- 
tion.    In  the  vertebral  column  of  Vertebrates,  for   example, 
we  have  a  structure  which  indicates  clearly  a  relationship 
between  the  five  classes  of  Vertebrates,  but  when  we  examine 
the  skull  and  the  appendages  we  seem  at  first  to' have  only 
hopeless    diversity.     But   here   also   a   wonderful   uniformity 
appears  on  more  careful  investigation.     With  regard  to  the 
skeletal  portions,  the  wing  of  a  bird,  the  fore  limb  of  a  bat,  the 
flipper  of  a  whale,  the  fore  legs  of  the  horse  and  dog  and  the  arm 
of  man,  are  all  constructed  of  the  same  elements  and  each  one 
of  these  appendages  may  be  homologized  bone  for  bone  with 


3QO  GENERAL   PRINCIPLES 

the  others.  It  must  be  kept  in  mind  that  bones  may  fuse,  may 
dwindle  by  degeneration  to  the  point  of  disappearance,  and  even 
new  bones  may  develop,  which  have  no  counterpart  in  other 
animals.  But  such  modifications  do  not  detract  from  the  value 
of  homologies.  In  the  same  way  the  skulls  of  these  five  classes 
are  also  found  to  belong  to  the  same  type,  and  one  can  homolo- 
gize  the  bones  of  a  frog's  skull  with  those  of  a  dog.  The  resem- 
blances between  Birds  and  Reptiles  are  especially  numerous  in 
parts  of  the  skeleton,  but  a  detailed  knowledge  of  comparative 
osteology  is  necessary  to  a  full  appreciation  of  such  points. 

793.  The  plant  kingdom,  as  a  whole,  does  not  form  as  perfect 
or  continuous  a  series  as  the  animal  kingdom,  but  all  the 
important  breaks  in  the  series  fall  below  the  Archegoniates. 
The  simple  structure  of  the  Algae  and  Fungi  offers  compara- 
tively few  features  for  comparison,  and  hence  makes  it  difficult 
to  discern  relationship.     On  the  other  hand,  the  series  from 
the  liverworts  to  the  highest  flowering  plants  is  more  perfect 
and  more  extensive  than  anything  to  be  found  among  animals. 
When  we  trace  the  gradual  development  of  the  sporophyte  and 
the  corresponding  reduction  of  the  gametophyte,  through  the 
various  classes  of  Bryophyta,  Pteridophyta  and  Spermatophyta 
we  are  able  to  form  an  almost  ideally  perfect  series.     When  one 
studies  this  series,  not  only  as  a  whole  but  in  its  details,  the  con- 
viction that  it  represents  a  " blood  relationship"  is  irresistible. 

794.  The  anatomist  often  discovers  organs  which  are  clearly 
without  function.     It  is  impossible  to  account  for  such  organs 
except  on  the  ground  that  they  are  rudiments  of  organs  which 
were  at  one  time  functional.     That  such  is  the  case  is  usually 
evident  by  comparison  with  other  species  in  which  homologous 
organs  are  to  be  found.     When  an  organ  becomes  useless  for 
any  reason  it  still  persists  in  a  more  or  less  imperfect  condition 
as  a  vestige  of  its  former  state.     Some  examples  of  vestigeal 
organs  are  of  particular  interest  in  the  present  connection.!" 

795.  The  whale  is  a  mammal,  since  it  suckles  its  young,  as 


RUDIMENTARY   ORGANS  39 1 

well  as  for  many  other  reasons.  But  it  has  only  two  appendages, 
the  flippers,  which  represent  the  fore  limbs.  No  evidence  of 
hind  limbs  is  to  be  found  externally,  but  on  dissection  a  rudi- 
mentary pelvic  girdle  may  be  found,  and  in  the  Greenland 
whale  there  are  also  rudimentary  femurs  and  tibias.  The 


FIG.  243. — The  "Congo  Snake,"  Amphiuma  means.  This  animal  is  not  a 
reptile  but  an  amphibian.  It  is  found  in  the  southern  United  States.  Note 
the  rudimentary  appendages.  Xi/3- 

Ophidia  are  classed  as  an  order  of  Reptilia,  but  they  have  no 
appendages.  Yet  in  the  case  of  the  python  there  is  a  rudi- 
mentary pelvis  and  also  rudimentary  appendages  which  appear 
at  the  surface  as  horny  points.  Most  lizards  have  two  pairs  of 


39 2  GENERAL  PRINCIPLES 

functional  appendages,  but  in  several  species  the  appendages  are 
more  or  less  rudimentary.  In  one  the  fore  limbs  are  entirely 
wanting,  while  the  hind  ones  are  greatly  reduced.  In  still  an- 
other form  there  are  no  limbs  at  all,  but  both  girdles  are  present. 
In  all  these  cases  we  have  animals  which  are  classed  with  quad- 
rupedal forms,  though  they  do  not  have  four  legs.  The  sig- 
nificance of  such  rudimentary  structures  cannot  be  overlooked. 


FIG.  244. — The  female  luna-moth,  Tropaea  lima,  seen  from  beneath.  The 
abdomen  becomes  so  heavy  before  the  eggs  are  deposited  that  the  moth  is 
unable  to  fly.  XL 

796.  The  young  baleen  whale  has  rudimentary  teeth  which 
never  develop  to  a  point  where  they  can  be  of  service  to  the 
animal.  In  the  middle  of  the  upper  surface  of  the  skull  of 
many  lizards  there  is  a  small  opening.  Over  this  place  the  skin 
is  transparent,  and  beneath  there  is  an  eye  which  is  connected 
with  the  fore  brain  by  a  long  stalk.  In  the  Cyclostomes  there 


RUDIMENTARY   ORGANS  393 

is  a  similar  eye  in  a  functional  condition,  but  in  all  other  Verte- 
brates it  is  wanting.  In  its  place  there  is  an  organ  whose  func- 
tion is  not  known  but  which  is  homologous  with  the  pineal  eye 
of  Cyclostomes  and  lizards,  and  is  probably  a  functionless 
rudiment.  The  rudimentary  paired  eyes  of  cave  fishes  belong 
in  the  same  category. 

797.  As  an  example,  from  among  invertebrates  the  wings 
of  insects  may  be  mentioned.  Most  insects  have  wings,  but 
the  order  Aptera  contains  no  winged  forms.  In  this  group 
there  are  no  rudimentary  wings  or  other  evidence  that  the 
insects  ever  possessed  wings.  The  order  Hemiptera  contains 


FIG.  245. — Hibernia  marginaria,  a  species  of  moth  in  which  the  female  has 
wings  much  reduced  and  useless.     A,  Male;  B,  female.     Xi  1/2. 

many  wingless  forms,  but  many  others  are  winged  and  many 
have  rudimentary  wings.  In  this  case  the  wingless  forms  are 
regarded  as  representing  a  degenerate  condition.  In  other 
orders  there  is  also  evidence  of  degeneration  of  wings.  The 
male  gipsy  moth  flies  well.  The  female  is  also  provided  with 
well-developed  wings,  but  she  never  uses  them.  Among  the 
species  of  the  geometrid  moth  genus,  Hibernia,  the  females  are, 
in  some  cases,  wholly  without  wings,  while  in  others  various 
stages  in  the  reduction  of  wings  may  be  found.  A  similar 
condition  exists  in  the  beetle  family,  Lampyridae,  the  common 


394  GENERAL  PRINCIPLES 

"fire  flies."  Here  the  males  are  always  good  fliers,  and  in  some 
species  the  females  are  also,  but  in  other  species  the  females 
have  rudimentary  wings  or  the  wings  are  entirely  wanting. 

798.  Among  plants  the  rudimentary  leaves  and  other  organs 
to  which  frequent  reference  has  been  made  are  further  examples 
of  the  principle  under  discussion. 

799.  In  the  first  series  of  examples  cited  in  this  section  we 
saw  how  organs  adapted  for  such  diverse  purposes  as  swimming, 
walking,  flying  and  writing  may  be  constructed  on  a  common 


FIG.  246. — Hibernia  defoliaria.     The  female  is  wingless.     See  preceding 
figure.     A,  Male;  B,  female.     Xi  1/2. 

plan.  No  plausible  explanation  for  this  remarkable  fact  has 
ever  been  offered  except  that  of  a  common  origin.  If  these 
animals  had  a  common  pentadactyl  ancestor  the  present 
diversity  as  to  the  condition  of  their  appendages  is  the  result 
of  modification  in  different  directions  as  a  result  of  different 
conditions  of  environment. 

800.  The  vestigeal  organs  also  can  only  be  accounted  for 
on  the  supposition  that  the  ancestral  forms  possessed  the  organs 
in  a  functional  condition,  and  that  changed  conditions,  involv- 
ing a  disuse  of  the  organs,  resulted  in  their  degeneration.  This 


THE   PHYLOGENETIC    SERIES  395 

means  then  also  that  the  rudimentary  organ  is  an  indication 
of  kinship  with  forms  in  which  it  exists  in  a  functional  condition. 

801.  If  this  principle  is  granted  the  kinship  of  the  entire 
organic  world  can  be  more  or  less  completely  established. 

802.  The  Phylogenetic  Series. — A  large  part  of  the  rocks 
of  the  surface  of  the  earth  were  formed  by  the  deposits  of  mud, 
sand  and  organic  remains  under  water.     Thus  originated  the 
shales,    sandstones,    and   limestones.     While   the   rocks   were 
being  deposited  the  bodies  of  animals  and  plants  were  frequently 
buried  and  the  resistant  parts  preserved  as  fossils.     Of  course, 
the  rocks  which  were  formed  first  are  now  beneath  those  which 
were  deposited  later.     By  the  fossils  and  their  relative  posi- 
tion in  the  rocks  we  have  been  able  to  learn  something  of  the 
character  of  the  former  inhabitants  of  the  earth  and  of  the 
order  in  which  they  appeared.     The  geologist  has  been  able  to 
reconstruct  in  considerable  detail  the  more  recent  periods  of 
the  earth's  history,  but  the  earlier  chapters  are  difficult  to 
decipher. 

803.  The  Cambrian  period  is  the  earliest  in  which  we  find 
any  evidence  of  life.     It  was  a  period  covering  a  vast  extent  of 
time,  and  at  its  close  most  of  the  invertebrate  phyla,  if  not  all, 
were  already  represented.     The  highest  Molluscs,  the  Cephalo- 
pods,  were  present,  and  also  the  aquatic  Arthropods,  the  Crus- 
tacea.    Insects  and  spiders  appeared  in  the  next  period,  the 
Silurian,  and  also  the  first  Vertebrates,  Fishes.    In  the  Devonian 
period  Fishes  were  extremely  abundant.     The  Amphibians  and 
Reptiles   appeared  in   the   Carboniferous  period.     Mammals 
did  not  appear  until  the  Jurassic  period,  and  Birds  came  still 
later,  in  the  Triassic.     So  far  as  this  evidence  goes  it  shows  that 
the  lower  forms  appeared  first.     With  regard  to  Vertebrates, 
more  particularly,  the  order  in  which  the  classes  appeared  also 
agrees  with  what  one  would  expect  according  to  the  theory  of 
descent. 

804.  The  fact  that  Birds  appeared  after  Mammals  must  not 


396 


GENERAL   PRINCIPLES 


be  misinterpreted.  Of  two  forms,  the  one  which  appeared  later 
is  not  necessarily  descended  from  the  other,  since  both  may 
have  arisen  independently  from  still  earlier  forms.  And  the 
one  which  appeared  latest  is  not  necessarily  the  highest.  Birds 
are  as  highly  specialized  as  Mammals.  But  the  mammalian 
type  of  specialization  may  be  described  as  a  more  successful 
one,  and  Mammals  are,  therefore,  usually  placed  above  Birds. 

805.  The  geological  record  is  very  fragmentary  and  only 
occasionally  do  we  get  a  connected  story.  The  history  of  the 
snail,  Paludina,  has  been  worked  out  in  detail,  and  we  have  an 


FIG.   247. — Fossil  remains  of  ArchaEOpteryx  lithographica. 
(From  Galloway,  after  Claus.) 


account  of  the  changes  through  which  the  genus  passed  during 
a  considerable  interval  of  time.  The  shell,  which  was  at  first 
very  simple,  with  smooth,  rounded  contours,  became  step  by 
step  angular  and  ribbed.  So  that  the  later  species  have  little 
resemblance  to  the  earlier  forms. 

806.  The  first  bird  of  which  we  have  any  record  is  the 
Archaeopteryx.  This  bird  had  a  long  tail  consisting  of  a  series 
of  vertebrae,  fringed  on  either  side  with  feathers.  The  wings 
were  provided  with  three  free  digits  armed  with  claws.  The 
head  was  very  large  and  had  heavy  jaws,  both  of  which  were 


THE   PHYLOGENETIC    SERIES 


397 


398  GENERAL  PRINCIPLES 

provided  with  a  complete  series  of  teeth.  Some  of  the  birds 
of  the  Cretaceous  period  were  much  more  like  modern  birds, 
but  still  had  teeth  on  certain  parts  of  the  jaws.  The  Archaeop- 
teryx  was  almost  as  much  reptile  as  bird,  and  even  the  later 
types  presented  many  decided  reptilian  characters. 

807.  The  most  complete  series  of  mammalian  fossils  are  those 
which  show  the  genesis  of  the  modern  horse.     The  earliest 
mammals  were  pentadactyl  (Phenacodus) .     The  earliest  horse- 
like  form  had  four  toes  on  the  fore  foot  and  three  on  the  hind 
foot.     From  this  we  see  the  number  of  toes  gradually  reduced, 
until  in  the  modern  horse  there  is  only  one  functional  digit  and 
small  rudiments  of  the  second  and  fourth.     The  first  digit  (I) 
was  the  first  to  disappear  and  then  the  fifth  (V).     These  were 
then  followed  by  the  second  and  fourth. 

808.  Geological    evidence    concerning    the    history    of    the 
development  of  the  vegetable  kingdom  is  much  like  that  for 
animals.     The  Cryptogams  existed  in  great  profusion  long  be- 
fore the  seed-bearing  plants  appeared,  and  the  Gymnosperms 
preceded  the  Angiosperms. 

809.  The  Ontogenetic  Series. — The  differences  between  indi- 
viduals are  the  greatest  when  the  individuals  are  mature.     The 
young,  the  late  embryos  and  the  earlier  embryonic  stages  are 
successively  more  and  more  alike,  and  differences  finally  vanish 
in  the  egg.     Hence  all  metazoa  start  from  the  same  level.     In 
fact,  the  differences  which  exist  up  to  the  end  of  gastrulation 
are  of  secondary  importance  and  have  no  relation  to  the  sys- 
tematic rank  of  the  developing  organism.     All  metazoa  are, 
therefore,  in  a  real  sense  alike  up  to  the  end  of  gastrulation. » 

8 10.  Suppose  an   observer  entirely  unacquainted  with  the 
characteristics  of  eggs  were  given  a  series  of  eggs  representing, 
we  will  say,  Ccelenterates,  Annelids,  the  lancelet,  a  fish,  a  bird 
and  a  rabbit.     In  the  eggs  themselves  the  observer  would  find 
no  means  of  determining  the  class  to  which  they  belonged.     If 
now  these  eggs  each  passed  through  its  appropriate  develop- 


THE    ONTOGENETIC   SERIES  399 

mental  processes  the  first  identification  would  be  possible  after 
gastrulation,  because  at  this  point  the  ccelenterate  development 
ceases.  The  other  embryos  continue  their  development  by  the 
formation  of  the  third  layer,  the  mesoderm.  The  body  becomes 
elongated  and  the  mesoderm  assumes  the  form  of  a  series  of 
mesodermic  somites  by  which  the  body  is  divided  into  a  homono- 
mous  series  of  metameric  segments.  Here  the  annelid  larva 
has  attained  the  form  of  the  adult  worm,  and  hence  the  end  of 
its  development.  The  other  larvae  develop  gill  slits,  and  at  this 
stage  the  development  of  the  lancelet  is  virtually  completed. 
The  further  addition  of  large  cerebral  vesicles  and  paired  eyes 
would  indicate  vertebrate  embryos,  the  fish,  the  bird  and  the 
mammal.  But  only  the  bird  and  the  mammal  would  develop 
lungs  and  pentadactyl  appendages.  In  the  bird  the  aortic 
arch  would  lie  on  the  right  side  of  the  body,  while  in  the  mammal 
it  would  be  on  the  left. 

811.  In  this  comparison  of  the  development  of  animals 
details  are,  of  course,  omitted.  The  purpose  of  the  comparison 
is  to  show  that  the  difference  in  the  result  of  development  of  the 
lower  and  higher  forms  is  due  not  to  a  difference  in  direction  of 
development  so  much  as  to  its  extent.  All  forms  pursue  the 
same  course,  but  the  higher  forms  continue  their  development 
farther.  Stating  the  same  thing  in  another  way,  we  may  say 
that  the  higher  forms,  in  their  development,  pass  through 
stages  which  are  the  permanent  adult  condition  of  lower  forms. 
Why?  If  it  is  granted  that  the  baleen  whale  is  descended 
from  a  terrestrial  quadruped  with  the  dentition  characteristic 
of  Mammalia,  then  the  rudimentary  teeth  of  the  young  whale 
are  a  relic  of  the  former  condition,  and  in  its  development  the 
whale  passes  beyond  the  tooth-bearing  stage  to  a  stage  of  rudi- 
mentary teeth.  This  argument  applies  to  all  rudimentary  or 
vestigeal  organs.  The  same  line  of  reasoning  may  be  applied 
to  another  type  of  development  in  which  an  organ,  instead  of 
remaining  rudimentary,  passes  beyond  the  normal  type. 


400  GENERAL  PRINCIPLES 

For  example,  the  wing  of  the  bat  is  supported  chiefly  by  the 
enormously  developed  phalanges  of  the  II,  III,  IV,  and  V 
digits.  In  the  embryo  the  bat  hand  is  at  first  a  normal  pen- 
tadactyl  hand,  and  the  great  elongation  of  the  four  fingers 
does  not  take  place  until  very  late.  The  bat  being  also  a 
Mammal  follows  the  mammalian  type  of  development,  and 
after  it  has  reached  the  grade  of  a  Mammal  it  continues  its 
development  into  the  bat  stage  by  transforming  the  mammalian 
appendage  into  the  more  specialized  bat  appendage. 

812.  If  the  above  is  a  true  conception  of  the  origin  of  vesti- 
geal  and  highly  specialized  organs  then  we  are  also  in  a  position 
to  understand  the  parallelism  of  development  described  above. 
It  has  been  stated  as  a  "Fundamental Law  of  Biogenesis"  that 
"the  development  of  the  individual  recapitulates  the  history  of 
the  race,"  which  means  that  in  its  development  each  organism 
passes  through  stages  which  represent  the  adult  condition  of 
ancestral  forms. 

813.  The  fish  has  four  or  five  pairs  of  gill  slits  and  between 
the  slits  are  the  gill  arches  which  bear  the  gills.     Farther  back 
in  the  mid-ventral  line  lies  the  heart,  from  which  a  vessel  runs 
forward  and  divides  into  as  many  pairs  of  vessels  as  there  are 
pairs  of  gill  arches.     These  vessels  go  one  to  each  gill  arch,  and 
above  as  many  vessels  pass  from  the  arches  to  the  mid-dorsal 
line,  where  they  unite  into  a  single  vessel,  the  dorsal  aorta. 
By  these  vessels  the  blood  passes  from  the  heart  over  the  gills 
for  respiration.     In  the  embryo  of  Amphibia,  Reptiles,  Birds 
and  Mammals,  we  also  find  these  gill  slits  and  the  same  arrange- 
ment of  blood  vessels.     In  Amphibia  this  fish-like  condition 
persists  in  a  functional  manner  until  the  time  of  metamorphosis 
of  the  tadpole.     But  in  the  other  three  classes  respiration  by 
gills  never  occurs  and  the  gill  slits  are  functionless  rudiments. 
For  the  arrangement  of  the  several  pairs  of  vessels  which  pass 
over  these  functionless  gill  arches  there  is  also  no  explanation 
to  be  offered  except  that  they  have  been  inherited  from  fish-like 


MAMMALIAN    GILL   ARCHES 


4OI 


ancestors.  The  blood  passes  through  these  vessels  for  a  time, 
but  soon  an  entire  rearrangement  takes  place,  and  in  Birds  and 
Mammals  the  connection  between  the  heart  and  dorsal  aorta 


FIG.  249. — Diagram  of  the  human  embryo  to  show  the  arrangement  of  the 
blood-vessels.  E,  Eye;  0,  ear;  Mn,  lower  jaw;  H,  heart.  From  the  heart  a 
large  vessel  leads  forward  and  then  divides  into  five  pairs  of  vessels  (only  the 
five  of  one  side  are  represented).  These  vessels  pass  over  the  gill  arches  to  the 
dorsal  side  and  there  unite  to  form  the  dorsal  aorta.  In  the  adult,  however, 
only  the  fourth  branch  of  the  left  side  remains  to  connect  the  heart  with  the 
dorsal  aorta.  (From  McMurrich  after  His.) 

is  maintained  only  by  a  single  vessel.  In  Birds  this  vessel 
lies  on  the  right  side,  in  Mammals  on  the  left.  The  other  vessels 
either  become  connected  with  other  parts  of  the  body  or  else 

26 


402  GENERAL    PRINCIPLES 

fail  to  develop  at  all.     In  adult  Amphibia  and  Reptiles  a  pair  of 
these  vessels  persists  to  form  the  aortic  arches. 

814.  The  conclusion  seems  evident.     The  gill  arches  and 
their  blood  vessels  are  a  fish  character,  and  their  presence  in  the 
terrestrial  vertebrates  can  only  mean  that  as  vestigeal  organs 
they  hark  back  to  fish-like  ancestors.     But  this  is  only  one  of 
many  anatomical  puzzles  which  can  be  explained  in  this  way. 

815.  The   remarkable   parallelism   which   appears   between 
the  Taxonomic  series,   the  Phylogenetic  series  and  the  On- 
togenetic  series  assuredly  warrants  the  formation  of  a  provis- 
ional   hypothesis    of  the  origin  of  species  by  descent.     The 
value  of  any  hypothesis  is  gauged  by  the  extent  to  which  it 
explains  phenomena  and  by  the  help  it  gives  in  the  discovery 
of  new  facts.     We  shall  proceed  to  apply  this  test,  but  let 
us   first    enquire    what  causes  might  be  supposed    to    bring 
about  a  change  in  species. 

8 1 6.  The  Struggle  for  Existence.— Taking  the  whole  world 
into  account  and  year  after  year,  there  is  on  the  average  no 
great  change  in  the  number  of  individual  organisms.     Locally 
and  for  brief  periods  there  frequently  occurs  an  increase  or  a 
decrease  in   the  number  of  a  given  species.     But  extended 
changes  of  this  kind  are  comparatively  rare  even  for  a  single 
species.     This  indicates  that  each  pair  of  adult  individuals 
at  the  time  of  death  have  provided  a  progeny  of  the  same  num- 
ber to  fill  the  gap.     But  the  rate  at  which  even  the  slowest 
breeding  animals  reproduce  is  much  in  excess  of  this,  and  for 
many  the  rate  is  many  thousand-fold  greater.     Many  plants 
produce  several  thousand  to  several  million  seeds  in  a  season 
and,  in  the  case  of  perennials,  this  is  done  for  many  years. 
For  many  animals  a  single  brood  of  eggs  ranges  from  many 
thousands  to  many  millions.     The  conger  eel  may  produce  five 
or  six  million  eggs,  while  the  female  Ascaris  is  credited  with  a 
brood  of  sixty-four  million  eggs.     Yet  in  these  cases  only  one 
or  two  eggs  can  ultimately  have  developed  into  an  individual 


SURVIVAL   OF   THE   FITTEST  403 

of  reproductive  age.  The  others  must  in  some  way  be  destroyed. 
Many  seeds  and  eggs  are  devoured  by  animals  and  many  are 
not  brought  into  an  environment  favorable  for  development. 
But  one  need  not  observe  very  closely  to  discover  that  the 
number  of  young  is  always  much  greater  than  the  number  of 
adults.  The  Nemesis  of  destruction  follows  the  young  through- 
out the  period  of  development,  and  indeed  throughout  life, 
but  the  ratio  of  mortality  is  greatest  during  the  earlier 
stages. 

817.  Now  what  is  it  that  determines  which  one  of  a  thousand 
young  should  survive?     Is  it  merely  a  matter  of  chance  or  is 
there  a  difference  between  individuals  which  gives  certain  ones 
an  advantage?    Let  us  consider  an  imaginary  concrete  example. 
Suppose  a  litter  of  young  rabbits  in  a  nest  at  the  edge  of  a  forest. 
These  young  are  more  or  less  like  their  parents  (heredity), 
but  are  not  all  exactly  alike  (individual  variation).     One  soon 
shows  itself  to  be  puny,  is  ill-nourished,  and  perhaps  falls  a 
prey  to  disease  or,  being  less  active,  is  the  first  to  fall  a  prey  to 
the  weasel  or  other  predacious  animal.     Its  more  active  mates 
escape  the  first  assault,  but  one  is  particularly  light  in  color 
and  is  readily  seen  at  night  in  the  open  field,  where  both  he 
and  the  owl  are  seeking  their  food.     Perhaps  one  is  too  dark  and 
his  color  fails  to  blend  with  the  dead  grass  where  he  attempts 
to  hide.     One  may  be  rather  stupid.     He  fails  to  sense  the 
enemy  until  it  is  too  late.     But  among  the  rest  there  is  one 
just  the  right  color,  that  of  his  successful  parents.     He  is  alert, 
strong  of  limb  and  a  nimble  dodger.     He  runs  fast,  dodges 
quickly  and  has  the  instinct  to  hide  at  the  right  time  and  place. 
He  is  the  one  most  likely  to  survive  to  the  season  when  he  estab- 
lishes a  family  of  his  own. 

8 1 8.  This  is  a  purely  imaginary  case,  but  no  unreasonable 
supposition  is  made.     That  rabbits  vary  in  regard  to  such 
characters  cannot  be  questioned,  nor  that  deficiency  in  such 
matters  may  be  fatal  to  the  individual.     The  matter  may  be 


404  GENERAL  PRINCIPLES 

summed  up  in  a  few  words.     It  is  the  fittest  that  survive  in 
the  struggle  for  existence. 

819.  Natural  Selection. — In  this  brood  of  rabbits  we  have 
imagined  a  process  of  natural  selection  to  take  place  by  which 
the  unfit  are  eliminated.  Since  the  parent  rabbits  succeeded 
they  must  have  been  fit  and,  therefore,  the  young  ones  which 
resembled  the  parents  closely  would  be  fit  also,  provided  they 
lived  under  the  same  conditions.  This  process  would,  there- 
fore, tend  to  preserve  the  type  of  the  parents.  But  conditions 
change  and  a  locality  which  at  one  time  is  most  favorable  for 
a  given  species  may  become  less  so.  Moreover,  species  often 
migrate.  If  a  locality  becomes  overcrowded  or  the  food 
scarce,  or  enemies  too  numerous,  there  is  a  special  impetus 
given  to  the  migrating  tendency.  Thus  a  species  may  push 
out  into  a  new  and  quite  different  environment  where  there  is 
a  different  nature  at  the  work  of  selecting.  Suppose  again 
the  rabbits:  They  have  pushed  out  into  colder  regions,  where 
the  snow  lies  on  the  ground  for  many  months.  The  gray  rabbit 
would  be  very  conspicuous  against  the  snow  and  a  coat  of  white 
fur  would  be  decidedly  advantageous  in  winter.  Hence  in- 
dividuals with  white  coats  in  winter  might  be  selected  here, 
while  in  other  regions  the  winter  gray  continues  to  hold  the 
advantage.  The  diverse  conditions  would  thus  tend  to  pro- 
duce two  varieties  of  rabbit  or  indeed  two  species,  the  winter 
white  and  the  winter  gray.  So  long  as  the  two  kinds  remain 
connected  by  intermediate  forms  they  could  be  only  called 
varieties,  but  if  the  intermediate  forms  disappear  they  would 
be  distinct  species.  This  is  not  intended  to  be  regarded  as 
an  explanation  of  how  the  two  species  of  rabbits  originated. 
It  is  simply  a  hypothetical  case  which  may  help  one  to  an  under- 
standing of  the  method  by  which  natural  selection,  acting 
through  individual  variation,  may  produce  new  species.  The 
process  of  natural  selection  must  necessarily  be  a  very  slow 
one,  and  there  are  few  historical  records  of  such  changes. 


NATURAL   SELECTION  405 

There  is,  however,  much  indirect  evidence,  which  we  will  now 
consider. 

820.  Animals  and  Plants  Under  Domestication. — All  our 
domestic  animals  and  plants  were  originally  wild  species.     They 
have  become  so  changed  under  domestication  that  most  of  them 
bear    little    resemblance    to    their   prototypes.     The    various 
types  of  fancy  pigeons,  the  carriers,  fantail,  tumbler,  pouter, 
etc.,  have  probably  all  been  produced  by  selective  breeding 
from'the  wild  rock  pigeon.     The  differences  in  structure  of  these 
fancy  pigeons  is  much  more  than  enough  to  give  them  specific 
standing.     Such  differences  found  in  wild  species  would  be 
regarded  as  of  generic  value.     How  did  they  come  about? 
Simply  by  selection.     The  breeder  selects  those  which  conform 
to  a  certain  type  and  thus  produces  a  "breed." 

821.  Our  dogs  may  be  descended  from  two  or  three  wild  dogs 
or  wolves,  but  the  original  type  has  little  in  common  with  the 
hundreds  of  breeds  of  dogs,  ranging  from  mastiff  to  greyhound 
and  from  poodle  to  St.  Bernard.     In  our  horses  and  cattle,  cats, 
poultry,    garden   vegetables   and   cereals   similar   remarkable 
effects  have  been  produced.     In  some  of  these  domestic  breeds 
selection  has  been  at  work  for  a  long  period,  but  often  marked 
results  have  been  brought  about  in  a  short  time. 

822.  The  conclusion  which  we  may  draw  from  the  facts  of 
varieties  under  domestication  is  that  species  are  not  immutable, 
and  if  man  by  selection  can  produce  such  results  it  is  reasonable 
to  believe  that  nature  by  some  process  may  bring  about  similar 
results.     The  natural  process  is  certainly  slower,  but  the  time 
during  which  it  has  been  at  work  is  vastly  longer. 

823.  Geographical  Distribution. — If   such   species   had   an 
independent  origin  (not  by  descent)  then  there  is  no  reason 
why  two  similar  species    should    be    related   geographically. 
They  may  as  well  occupy  islands  on  the  opposite  sides  of  the 
globe,  provided  food,  climate,  etc.,  are  the  same,  as  live  in  adja- 
cent countries.     If,  however,  two  species  had  a  common  origin 


406  GENERAL   PRINCIPLES 

(by  descent)  they  must  also  be  related  geographically;  either 
they  still  live  in  the  land  where  they  originated  or  else  there 
must  have  been  a  path  along  which  they  could  migrate  to  the 
place  where  they  are  now  found. 

824.  Hence,  the  present  geographical  distribution  of  animals 
may  throw  light  on  the  question  of  the  origin  of  species. 

825.  Australia  is  separated  from  Asia  by  a  barrier  which  is 
practically  impassable  for  mammals,  and  geologists  tell  us  that 
this  has  been  the  case  for  a  long  time — since  the  Cretaceous 
period.     Now  if  species  change  in  the  course  of  time  one  would 
expect  to  find  the  Australian  mammals  unlike  those  of  Asia  or 
elsewhere.     This  expectation  is  fulfilled  in  a  remarkable  way. 
All  the  mammals,  except  a  few  which  we  have  reason  to  believe 
were  carried  there,  are  Monotremes  and  Marsupials.     These 
are  the  most  primitive  mammals  and  no  living  forms  are  found 
outside  of  the  Australian  region  except  the  American  opossum. 
Fossil  remains  show  that  Marsupials  were  at  one  time  wide- 
spread, but  evidently  they  were  unable  to  contend  with  the 
higher  mammals  and  became  extinct.     In  Australia  no  higher 
mammals    developed.     On    the    other    hand    the  Marsupials 
developed  in  great  variety;  herbivores,  carnivores,  gnawers, 
subterranean  mole-like  forms  and  tree-dwelling  forms. 

826.  Africa  and  South  America  are  also  somewhat  isolated, 
and  here  we  also  find  peculiar  faunas.     It  is  not  remarkable 
that  a  peculiar  species  of  any  kind  should  be  confined  to  a  given 
area,  but  where  several  similar  species  of  a  remarkable  genus  are 
found  in  the  same  isolated  region,  and  when,  farther,  fossils  of 
still  other  related  forms  are  found  in  the  same  region,  only  the 
hypothesis  of  a  common  origin  offers  a  satisfactory  explanation. 
Numerous  examples  of  this  kind  occur.     Some  examples  are 
the  following:  The  kiwi-kiwis  of  New  Zealand,  the  catarrhine 
monkeys  of  the  Old  World  and  the  platyrhine  monkeys  of  the 
New  World,  and  the  rheas  of  South  America.     The  Edentates 
of  the  Old  and  New  Worlds  are  also  of  distinct  orders. 


GEOGRAPHICAL   DISTRIBUTION  407 

827.  The  fauna  of  isolated  oceanic  islands  contributes  the 
same  kind  of  evidence.     Madagascar  has  many  species  found 
nowhere  else,  but  these  species  are  more  like  those  of  the 
neighboring  African  coast  than  those  of  more  remote  regions. 
The  Galapagos  Islands  also  have  a  peculiar  fauna,  which  finds 
its  greatest  affinity  in  the  fauna  of  the  nearest  part  of  the  South 
American  coast.     The  fauna  of  the  Azores  is  related  to  that  of 
Europe,   while   that   of   the   Bermudas   belongs   to   America. 
These  resemblances  of  faunas  cannot  be  attributed  to  the  effect 
of  food  and  climate  or  other  external  causes,  for  there  is  often 
a  greater  difference  in  environment  between  two  neighboring 
localities  than  between  others  on  opposite  sides  of  the  globe. 

828.  The  Hawaiian  Islands  are  very  mountainous  and  the 
mountains  are  cut  by  numerous  deep  parallel  valleys.     In 
these  valleys  are  found  numerous  species  of  the  snail,  Achati- 
nella.     The  snails  cannot  cross  the  mountain  barriers  and  hence 
there  is  little  migrating  of  snails  from  valley  to  valley.     The 
fact  that  almost  every  valley  has  its  own  peculiar  type  of 
snail,  and  the  way  these  species  are  distributed  on  the  island, 
makes  it  seem  probable  that  all  had  a  common  origin  and 
that  each  species  originated  where  it  is  now  found.     If  at  any 
time,  in  any  valley,  a  new  character  appears  through  individual 
variation  that  character  may  in  time  be  transmitted  to  all  the 
individuals  of  that  valley,  and  hence  become  a  specific  char- 
acter, but  natural  barriers  will  prevent  its  transmission  to  other 
species  living  in  other  valleys. 

ADAPTATIONS 

829.  In  Part  I  many  examples  of  modification  of  the  type 
structures  were  described  and  it  was  shown  that  these  modifi- 
cations were  always  associated  with  peculiarities  of  life  habit 
or  of  environment.     Such  modifications  of  an  organism,  in 
connection  with  peculiarities  of  the  condition  of  life,  are  known 


408  GENERAL   PRINCIPLES 

as  adaptations.  It  is  a  phenomenon  not  confined  to  plants. 
The  peculiarities  of  parasitic  animals  or  of  sedentary  animals 
are  also  adaptations.  In  fact,  the  sum  total  of  characteristics 
of  living  things  is  an  adaptation  to  conditions.  At  various 
points  in  this  book  the  idea  has  been  expressed  that  an  organ- 
ism is  what  it  is  because  of  the  conditions  under  which  it  lives. 
For  example,  one  often  hears  stated  that  foliage  is  green  because 
that  color  is  least  irritating  to  the  human  eye.  The  same  idea 
is  expressed  in  other  forms,  but  the  position  or  point  of  view  is 
entirely  false.  The  color  of  vegetation  is  part  of  the  environ- 
ment, and  natural  selection  would  result  in  the  development  of 
eyes  which  were  adapted  to  that  color. 

830.  There  are  many  special  types  of  adaptations  which 
are  of  interest  because  they  give  us  an  insight  into  the  method 
of  operation  of  natural  selection.     We  will  call  attention  to  a 
few  of  these  here. 

831.  The  dispersal  larvae  so  often  found  among  marine   ani- 
mals may  be  regarded  as  an  adaptation  by  which  the  species 
make  use  of  oceanic  currents  to  secure  transportation  from 
place  to  place.     The  various  devices  employed  by  plants  to 
secure  the  scattering  of  their  seeds  fall  in  the  same  category. 
On  the  other  hand  a  free-swimming  larva  would  be  fatal  to 
many  fresh-water  animals  because  the  larvae  would  be  carried 
to  the  sea  and  perish  in  the  salt  water.     The  eggs  of  very 
many  marine  fishes  are  light  and  float  freely  in  the  water,  but 
the  eggs  of  fresh-water  fishes  are  either  attached  by  means  of 
adhesive  secretions  or  else  are  so  heavy  that  they  lie  on  the 
bottom  among  the  pebbles  which  protect  them  from  the  cur- 
rent.    Many  fresh-water  fishes  make  nests  by  excavating  the 
bottom  and  the  eggs  are  often  covered  by  a  layer  of  pebbles. 
Some  marine  fishes  also  attach  their  eggs  or  construct  nests,  but 
the  habit  is  not  general. 

832.  The  eggs  of  the  decapod  Crustaceae  are  usually  attached 
to  the  abdomen  of  the  female,  but  in  the  marine  forms  the 


ADAPTATIONS  409 

young  hatch  early  and  are  then  set  free,  while  the  young  of  the 
crayfish  hatch  later  and  cling  to  the  mother  for  some  time  after 
hatching. 

833.  The  Amphibia  generally  deposit  their  eggs  in  water, 
but   they  avoid   the   streams  which  have  a  strong  current, 
preferring  quiet  pools,  ponds  or  even  stagnant  puddles.     They 
also  often  attach  the  eggs  to  objects  under  water  by  means  of 
the  gelatinous  envelope^  which  holds  the  eggs  together  in  masses. 

834.  The  embryo  of  the  marine  Lamellibranchs  is  set  free 
at  an  early  stage  as  a  free-swimming  veliger  (page  338).     At  a 
corresponding  stage  the  young  glochidium  of  the  fresh-water 
mussels,  the  Unios  and  Anodontas,  become  attached  to  fishes, 
where  they  continue  their  development  for  perhaps  several 
months  longer  before  they  finally  become  free. 

835.  Lakes   and  ponds  often  swarm  with  many  kinds  of 
minute   free-swimming   organisms,    which   are   comparatively 
rare  in  streams.     Only  the  larger  forms,  with  their  stronger 
swimming  powers,   can  make  headway  against  an  ordinary 
current,  and  are  thereby  enabled  to  maintain  themselves  in 
the  waters  of  creeks  and  rivers. 

836.  The  insect  faunas  of  oceanic  islands  present  a  similar 
phenomenon.     These  insects  are  either  wingless  or,  if  they 
have  wings,  are  seldom  seen  to  use  them.     The  explanation 
offered  is  very  simple.     Few  insects  are  able  to  fly  against  a 
strong  wind,  and  strong  winds  are  particularly  prevalent  on 
oceanic  islands.     Under  such  circumstances  if  an  insect  were 
to  rise  into  the  air  it  would  most  likely   be   carried   to   sea 
and  perish.     As  a  result  only  those  insects  which  cannot,  or  at 
least  do  not,  fly  have  remained. 

837.  Adaptations  to  Water. — Attention  may  again  be  called 
to  the  important  adaptations  which  have  reference  to  water. 
These  are  particularly  well  exemplified  by  comparing  Hydro- 
phytes and  Xerophytes,  or  by  comparing  aquatic  and  terrestrial 
animals,  especially  with  regard  to  the  integument. 


410  GENERAL   PRINCIPLES 

838.  Adaptations  with  regard  to  light  are  much  more  gen- 
eral and  important  among  plants  than   among  animals.     As 
touching  plants  the  matter  has  already  been  fully  discussed. 
Animals  are  much  less  dependent  on  light,  and  adaptation  with 
regard  to  light  affects  chiefly  the  eyes.     Animals  adapted  to 
absolute  darkness,  such  as  the  fishes,  salamanders  and  cray- 
fishes of  caves  have  usually  little  or  no  pigment  in  the  skin. 
The  most  striking  peculiarity  of  these  animals  is  that  they  are 
blind  and  the  eyes  are  usually  very  rudimentary.     The  tactile 
organs,  however,  are  better  developed  than  are  those  of  the 
normal  type.     This  is  especially  true  of  the  antennae   of  the 
crayfishes.     Blind  fishes  are  found  in  caves  in  many  parts  of 
the  world,  and  they  "belong  to  many  different  families,  but  are 
always  related  (similar)  to  the  forms  living  in  nearby  streams." 
This  fact  is  strong  evidence  that  the  blind  forms  have  descended 
from  the  ancestors  of  those  which  now  live  in  the  surface 
streams. 

839.  Adaptations  to  Changes  of  Temperature. — All  plants 
and  most  animals  are  directly  dependent  on  the  temperature 
of  the  surrounding  medium,  so  that  growth  and  other  vital 
processes  proceed  more  or  less  rapidly  in  accordance  with  the 
changes  in  temperature  of  the  surrounding  water  or  air.     The 
time  required  for  a  frog  to  develop  from  the  egg,  for  example, 
may  vary  from  seventy  days  at  a  temperature  of  60°  F.  to  two 
hundred  and  thirty  days  at  51°  F.     The  temperature  of  sea 
water  seldom  passes  the  limits  within  which  vital  processes 
are  possible.     The  temperature  of  the  air  varies  much  more 
widely,  and  consequently  terrestrial  organisms  present  several 
types  of  special  adaptations  with  regard  to  temperature.     All 
the  vital  activities  of  all  terrestrial  plants  and  animals,  except 
birds  and  mammals,  are  suspended  when  the  temperature  falls 
to  or  below  freezing.     Insects,  Amphibia  and  Reptiles  usually 
hide  in  sheltered  recesses  at  such  times  and  remain  dormant  until 


BODY   TEMPERATURE  411 

the  temperature  rises  again  to  a  point  which  will  permit  the 
organs  to  perform  their  functions. 

840.  Birds  and  Mammals  present  a  special  adaptation  in 
this  regard  in  the  comparatively  high  and  constant  temperature 
maintained  by  the  body.     This  is  done  by  the  expenditure  of  a 
considerable  amount  of  energy,   specifically  for  keeping   the 
body  warm,  and  for  this  reason  Birds  and  Mammals  require 
considerably  more  food  than  do  other  animals  of  corresponding 
size.     The  temperature  of  the  body  is  kept  constant  by  a 
control  mechanism  by  which  the  amount  of  heat  lost  is  con- 
trolled.    Much  heat  is  lost  by  evaporation  of  moisture  from 
the  lungs  and  respiratory  passages  and  from  the  mouth  cavity 
also  when  the  animal  is  panting.     Panting  greatly  increases 
the  amount  of  vaporization  and  the  accompanying  loss  of  heat. 
The  feathers  and  hair,  when  lying  close,  prevent  the  loss  of 
heat,  but  they  may  be  raised  on  end  by  the  action  of  special 
muscles  in  the  skin.     This  permits  free  circulation  of  air  and 
increases  the  loss  of  heat  by  radiation,  convection  and  vaporiza- 
tion.    The  skin  of  Mammals  is  provided  with  numerous  tubu- 
lar glands  which  discharge  their  secretion  on  the  surface.     This 
secretion  is  chiefly  water,  which  evaporates  from  the  surface  as 
rapidly  as  it  is  formed,  or  may  accumulate  in  small  droplets  of 
perspiration.     The  function  of  the  sweat  glands  seems  to  be 
primarily  temperature  control. 

841.  The  heat  of  the  body  is  distributed  by  the  blood.     The 
amount  of  blood  brought  to  the  surface,  and  there  cooled,  is 
regulated  by  the  expansion  and  contraction  of  the  blood  vessels 
of  the  skin.     The  supply  of  blood  to  the  skin  and  the  activ- 
ity of  the  sweat  glands  are  both  controlled  through  nervous 
stimulation. 

842.  Some  Mammals  have  in  a  measure  reverted  to  the  primi- 
tive   condition   of   variable   temperature.     Bears   and   many 
others  pass  a  considerable  portion  of  the  winter  in  a  deep  sleep, 
during  which  the  temperature  of  the  body  falls  to  a  low  point 


412  GENERAL   PRINCIPLES 

and  all  the  vital  processes  are  at  a  low  ebb.  This  sleep  is 
called  hibernation.  It  permits  the  animal  to  tide  over  the  sea- 
son when  food  is  difficult  to  find.  The  small  amount  of  energy 
required  to  maintain  life  in  the  hibernating  condition  is  fur- 
nished by  the  reserve  store  in  the  form  of  fat  which  the  animal 
possesses  at  the  beginning  of  winter. 

843.  Most  Birds  have  adapted  themselves  to  the  changing 
seasons  in  another  way.     At  the  approach  of  the  winter  season 
they  move  southward  in  easy  flights  of  twenty-five  to  fifty 
miles  a  day  and  thus  keep  south  of  the  region  of  ice.     In  the 
spring  this  migration  is  repeated  in  the  opposite  direction.     We 
do  not  know  what  impels  the  birds  to  begin  their  migration, 
for  they  do  not  wait  until  the  season  has  advanced  far  enough 
to  make  conditions  uncomfortable.     Nor  do  we  know  by  what 
means  the  bird  is  informed  in  which  direction  to  fly.     We  call 
such  actions  instinctive,  which,  however,  does  not  explain  them. 
It  may  be  that  they  should  be  classed  with  such  rhythmical 
physiological  processes  as  the  fall  of  the  leaves  of  deciduous 
trees,  and  tropisms  like  geotropism  and  heliotropism.     But 
whether  the   fact  is  explained  or  not  the  real  fact  remains, 
and  if  a  bird  failed  to  migrate,  that  bird  would  probably  not 
survive    the    winter    and    its    eccentricities    would    not    be 
perpetuated. 

844.  Adaptations  for  securing  food  are  exceedingly  manifold. 
Under  this  class  would  fall  most  of  the  peculiarities  connected 
with  saprophy  tic  and  parasitic  habits.     Insectivorous  plants,  the 
teeth  and  digestive  tract  of  the  carnivorous  and  herbivorous 
animals,  the  claws,  beak  and  digestive  tract  of  the  birds  of  prey, 
and  the  digestive  tract  of  grammivorous  birds  may  be  cited 
in  this  connection.     The  great  baleen  whale  feeds  on  minute 
pelagic  organisms,  which  it  secures  by  filling  its  mouth  with  the 
water  containing  the  food  and  then  straining  out  the  food  by 
allowing  the  water  to  flow  out  through  the  fringe  of  horny 
baleen  fibres  which  hangs  down  from  the  upper  jaw.     Many 


ADAPTATIONS 


413 


spiders  spin  a  web  which  entangles  small  insects  which  come 

in  contact  with  it.     Many  other  animals  set  traps.     Some 

aquatic  insect  larvae  construct  a 

trap  like  a  fish  net  and  these  open 

up  stream,  so  that  the  current  may 

sweep    small    edible  objects  into 

them.     The  angler  fish  lies  on  the 

bottom,  of  ten  very  much  concealed. 

His  most  capacious  mouth  opens 

upward   and   above   it   hangs  an 

attractive-looking  bait,   which  is 

phosphorescent    in     some    cases. 

When  the  prey  approaches  near 

enough    the    great   mouth   opens 

suddenly  and  the  rush  of  water 

into  it  carries  the  prey  along.     An 

African  snake  (Dasy-peltis)    feeds 

largely  on  eggs,which  are  swallowed 

whole.     Some  of  the  vertebrae  have 

pointed   processes   which   project 

into  the  oesophagus.     The  shells  of 

the  eggs  are  broken  against  these 

points  and  the  empty  shell  is  then 

disgorged. 

845.  Pollination. — Some  of  the 
adaptations  of  plants,  with  regard 
to   pollination,   have  been  noted 
elsewhere,   but    there   are    many 
special   cases  which   are  very  re- 
markable.    We    can     only    note 
briefly  a  few  of  them. 

846.  The   papilionaceous    bios-    which  the  f00d  is  strained  out  of 
som    of    most    members    of    the    the  water- 

pea  family  is  familiar  to  everyone.     This  flower  in  its  prime 


FIG.  250. — Part  of  one  of  the 
horny  baleen  plates  ("  whalebone  ") 
which  hang  from  the  upper  jaw  of 
the  baleen  whale.  One  edge  of  the 
plate  is  split  up  into  a  fringe  of 
hairs  which  form  the  filter  through 


GENERAL   PRINCIPIES 


has  its  axis  horizontal.  The  upper  petal  (standard)  is  very 
broad  and  stands  erect,  making  the  flower  very  conspicuous. 
The  two  lateral  petals  (wings)  form  a  horizontal  platform  upon 


FIG.  251. — A  row  of  the  nets  woven  by  the  Caddice  fly  larvae,  to  catch  food. 
In  the  next  figure  three  of  the  nets  are  seen  from  above. 

which  an  insect  (bee)  may  conveniently  rest  when  visiting  the 
flower  for  nectar.  The  two  lower  petals  are  slightly  curled  longi- 
tudinally and  have  their  concave  faces  toward  each  other  (keel) , 
so  that  they  completely  enclose  the  stamens  and  pistil.  The 
filaments  of  the  ten  stamens  are  all  united,  except  the  upper 


FIG.  252. — Nets  of  Caddice  worm.     See  Fig.  251. 

one,  for  most  of  their  lengths.  The  ends  only  are  free  and 
bend  upward.  The  style  also  bends  upward  at  the  end. 
When  an  insect  like  a  bee  visits  the  flower  its  weight  presses  the 


POLLINATION 


415 


wings  and  keel  down,  but  the  rigid  filament  tube  holds  the  stamens 
and  style  in  place.  The  upper  edges  of  the  keel  petals  separate 
and  the  anthers  come  into  view  and  touch  the  ventral  surface 
of  the  abdomen  of  the  insect,  leaving  upon  it  a  charge  of  pollen. 
The  insect  now  visits  another  flower  and  takes  the  same  posi- 


FIG.  253. — Bumblebee  (Brombus)  pushing  his  way  under  the  stigma  and  stamen 
of  the  blue  flag  (Iris  versicolor).     See  following  figure.     (From  Folsom.) 

tion  on  the  wings.  The  stigma  touches  the  same  part  of  the 
insect  which  before  came  in  contact  with  the  stamens  and  is 
consequently  covered  with  pollen  which  came  from  another 
flower. 

847.  In  some  of  the  mint  family  (Salvia)  the  flowers  are 
somewhat  funnel-shaped,  with  two  stamens  attached  to  the 


416 


GENERAL  PRINCIPLES 


corolla  near  the  opening  of  the  funnel.  Each  anther  is  at- 
tached to  the  upper  end  of  a  long  lever,  which  is  pivoted  in  the 
middle.  When  an  insect  enters  the  flower  it  brushes  against 
the  lower  end  of  this  lever,  causing  it  to  rise,  and  the  opposite 
end  with  the  anther  comes  down  on  the  insect's  back,  leaving 


FIG.  254.— Diagram  to  explain  the  preceding  figure.  The  bee  alights  on  the 
spreading  lobe  of  the  perianth  (s)  and  forces  his  way  under  the  stigmatic  shelf 
(/)  and  the  anther  (an).  In  doing  so  some  of  the  pollen  left  on  his  back  from  a 
flower  previously  visited,  is  scraped  off  by  the  stigma  and  then  he  is  immediately 
dusted  with  pollen  again  by  contact  with  the  anther.  The  nectary  is  shown 
at  n. 

a  dab  of  pollen  upon  it.  At  this  time  the  stigma  is  high  up 
under  the  hood-like  edge  of  the  corolla,  but  later  it  grows  out 
and  down  so  that  it  assumes  approximately  the  place  where  the 
anther  strikes  the  insect.  When  now  a  bee  which  has  previously 
been  dusted  with  pollen  visits  this  flower  the  stigma  brushes 
against  its  body  and  is  pollinated. 


POLLINATION  417 

848.  Some   of   the   orchids   present   the   most   remarkable 
adaptations  for  pollination   through   the  agency  of  insects. 
In  Arethusa  the  pollen  is  contained  in  a  receptacle  which 
opens  by  a  lid.     This  lid  is  torn  open  by  the  insect  in  its 
efforts  to  back  out  of  the  flower,  and  the  pollen  falls  upon  its 
back.     In  backing  out  of  the  flower,  however,  the  insect  first 
brushes  against  the  stigma,  which  would  then  be  pollinated, 
provided  the  insect  had  previously  visited  another  similar 
flower. 

849.  The  little  showy  orchid  (Galeorchis)  has  two  pollen  masses 
(pollinia)  which  lie  in  the  throat  of  the  corolla,  one  on  either 
side  of  the  stigma.     Each  pollen  mass  consists  of  pollen  grains 
bound  together  by  threatts  and  is  attached  to  a  sort  of  stalk 
which  ends  in  a  viscid  disc.     The  polliriium  is  enclosed  in  a  sack, 
but  the  disc  is  exposed  and  projects  forward  toward  the  entrance 

o  the  corolla.  When  an  insect  thrusts  its  head  into  the  throat 
of  the  corolla,  as  it  must  in  order  to  reach  into  the  deep  nectary, 
the  discs  of  the  pollinia  adhere  to  the  eyes  or  some  other  part 
of  the  head  and  are  withdrawn  when  the  insect  leaves  the 
flower.  The  position  of  the  pollinia  is  now  such  that  when 
another  similar  flower  is  visited  by  the  insect  the  pollinia  are 
thrust  directly  upon  the  broad  stigmatic  surface. 

850.  The  lady  slipper  (Cyprepedium)    has  again   another 
device.     The  large  cup  formed  by  the  "lip"  of  the  corolla  is 
readily  entered,  but  exit  is  difficult  because  of  the  way  the  edges 
are  inrolled.     A  small  opening  on  either  side  of  the  column, 
which  bears  the  two  anthers  and  the  stigma,  attracts  the  atten- 
tion of  the  prisoner  and  he  forces  his  way  through  one  of  these. 
In  doing  so  he  must  creep  under  the  column  and  his  back 
brushes  against  the  stigma  first  and  then  the  anther.     If  he 
had  previously  visited  a  similar  flower  some  of  the  pollen  on 
his  back  would  now  adhere  to  the  stigma  and^a  new  supply 
of  pollen  would  immediately  be  obtained  as  he  passes  the  anther, 
for  the  next  flower  visited. 

27 


4i8 


GENERAL  PRINCIPLES 


851.  The  flower  of  the  common  milkweed  (Asclepias)  is 
greatly  modified.  The  five  stamens  are  all  united  around  the 
ovary  and  they  alternate  with  five  funnel-shaped  nectaries. 
Externally  nothing  can  be  seen  of  either  anthers  or  stigmas, 
but  alternating  with  the  five  nectaries  are  five  narrow  slits 
which  open  slightly  at  the  upper  end  of  the  pendant  flowers. 
These  slits  open  into  the  stigmatic  cavities.  The  pollen  is  not 
powdery  but  adheres  in  masses  similar  to  the  pollinia  of  Galeor- 


6 


FIG.  255. — Structure  of  the  milkweed  flower.  (Figures  A  and  B  should  be 
inverted.)  A,  The  whole  flower;  B,  the  upper  part  of  A  enlarged;  C,  corolla; 
/,  slit  opening  into  the  stigmatic  cavity;  h,  nectary.  C,  a  pair  of  pollen  masses 
connected  by  a  V-shaped  appendage  and  the  sticky  disc  (d).  (From  Folsom.) 

chis.  There  are  five  pairs  of  such  pollen  masses  and  the  pairs  are 
united  in  the  form  of  a  letter  V,  with  a  sticky  substance  at  the 
point  of  the  V.  The  apex  of  the  V  coincides  with  the  lower  end 
of  the  slit  and  the  pollen  masses  are  embedded  in  pockets  which 
extend  upward  on  either  side  of  the  stigmatic  cavity.  When  the 
bee  is  clinging  "head  down''  to  the  pendant  flower  his  feet 
readily  slip  into  the  slits  and  are  thereby  guided  to  the  sticky 
apex  of  the  V  pollen  mass.  With  a  strong  pull  the  foot  is  re- 
leased, with  the  pollen  masses  attached.  When  later  the  same 


POLLINATION 


419 


foot  slips  into  a  similar  slit  the  pollen  masses  are  drawn  into  the 
stigmatic  cavity,  and  in  part  or  wholly  torn  off  by  the  struggle 
of  the  insect.  Occasionally  an  insect  is  not  strong  enough  to 
free  itself  from  these  traps  and  perishes,  suspended  by  one  or 
several  feet.  This  is  the  only  possible  method  of  pollination  in 
milkweeds. 

852.  The  flowers  of  the  Yucca  are  pollinated  by  the  Pronuba 
moth.  The  moth  deliberately  collects  the  pollen  with  the  fore 
feet,  then  goes  to  the  pistil  of  the  same  flower,  pierces  it  with 
her  ovipositor  and  deposits  an  egg.  She  then  goes  to  the  fun- 


FIG.  256. — A  wasp,  Sphex  ichneumonea,  with  a  number  of  the  milk  weed  pollen 
masses  attached  to  its  feet. 

nel-like  stigma  and  deposits  the  pollen.  This  operation  is  re- 
peated in  the  same  and  neighboring  flowers  until  her  eggs  are  all 
deposited.  The  carrying  of  pollen  to  the  stigma  is  an  indirect 
method  of  providing,  food  for  the  young,  since  it  causes,  the 
ovules  to  develop,  and  these  are  devoured  by  the  larvae  of  the 
moth  after  hatching.  When  ready  to  pupate  the  larvae  escape 
from  the  ovary  by  a  hole  which  they  bore  through  its  wall. 
Not  all  the  developing  seeds  are  devoured.  The  plant  sacri- 
fices a  part  of  the  seeds  as  a  reward  for  the  services  of 
pollination. 

853.  It  is  not  only  the  plants  that  are  affected  by  the  adapta- 


420 


GENERAL  PRINCIPLES 


tion  for  pollination.  The  insects  are  often  as  much  modified. 
The  worker  bee  has  special  organs  for  carrying  pollen,  the  Moth 
has  a  long  proboscis  for  reaching  into  the  extremely  deep  necta- 
ries of  certain  flowers  and  thePronubahas  developed  a  special  in- 
stinct for  pollinating  the  Yucca,  which  is  as  much  a  part  of  the 
insect  as  are  its  legs  and  wings.  In  many  cases  both  plant  and 
flower  are  so  modified  with  respect  to  each  other  that  one  can- 
not exist  without  the  other. 


FIG.  257.  FIG.  258. 

FIG.  257. — Pronuba  yuccasella  in  the  flower  of  the  Yucca.  (From  Folsom 
after  Riley.) 

FIG.  258. — Female  Pronuba  getting  pollen  from  the  anther  of  Yucca.  (From 
Folsom  after  Riley.) 

854.  Care  of  Young. — The  food  stored  up  in  seeds  and  in  eggs 
is  not  the  only  kind  of  provision  made  for  the  young  of  the  next 
generation.  Birds  feed  the  young  until  they  are  able  to  hunt 
food  for  themselves,  and  the  young  of  Mammals  are  fed  for  some 
time  from  the  secretions  of  the  mammary  glands  of  the  female. 
But  many  more  special  adaptations  occur.  To  mention  only 
one  from  the  plants:  The  mangrove  trees  grow  in  shallow 
water,  but  the  seeds  are  not  allowed  to  fall  and  drown  or  be 


CARE    OF   YOUNG 


421 


FIG.  259. — The  developing  seedling  of  the  mangrove.  In  i,  2  and  3,  the 
fruit  is  still  hanging  on  the  branch  but  the  hypocotyl  grows  until  it  reaches  a 
length  of  about  1 2  inches  when  it  drops  to  the  earth  and,  striking  in  the  mud, 
remains  standing  upright,  as  if  planted  (4)-  At  a  later  stage  (5)  roots  have  been 
developed  at  the  lower  end  and  an  epicotyl  from  the  upper  end. 


422  GENERAL   PRINCIPLES 

covered  up  with  the  mud  or  washed  away.  The  seed  remains 
on  the  tree  until  it  has  germinated  and  developed  a  heavy 
hypocotyl  about  a  foot  in  length.  It  then  falls  and  strikes  deep 
enough  into  the  mud  so  that  it  remains  upright  as  if  planted. 

855.  Among  the  social  Hymenoptera  the  young  are  cared  for 
as  carefully  as  they  are  by  the  higher  vertebrates.     In  other 
cases,  however,  the  young  are  never  seen  by  the  mother,  and, 
indeed,  in  many  cases,  the  mother  dies  before  they  are  hatched. 
But  even  in  such  cases  the  mother  may  make  elaborate  provision 
for   the  young.     The  Pronuba  will  illustrate  this  point,  but 
another  more  remarkable  example  is  frequently  quoted.     Many 
of  the  solitary  wasps  (Sphegidae)  excavate  channels  under  ground 
or  build  mud  nests  under  the  eaves  of  houses.     These  nests  are 
then  filled  with  spiders  or  other  insects  which  the  wasp  stings  in 
such  a  fashion  that  they  are  paralyzed,  but  not  killed.     An  egg 
is  then  deposited  by  the  wasp  and  the  nest  sealed  up.     The 
larva  soon  hatches  and  feeds  upon  the  spiders.     If  the  spiders 
had  been  killed  decomposition  would  soon  set  in  and  the  result 
would  probably  be  the  death  of  the  wasp  larva.     If  the  spiders 
were  not  sufficiently  stupefied  they  would  probably  kill  the  larva. 
It  is,  therefore,  of  great  importance  that  the  spiders  should  be 
stung  in  a  very  particular  manner.     But  the  wasp  never  returns 
to  the  nest  and  cannot  know  how  it  fares  with  her  offspring. 
If,  however,  her  work  was  not  well  done  she  will  have  no  off- 
spring to  inherit  her  careless  ways.     Our  wonder  and  admira- 
tion of  the  instinct  and  skill  of  the  successful  wasp  are  increased 
when  we  consider  that  the  proper  stinging  of  the  spiders  is  not  a 
deed  that  is  performed  with  calm  deliberation,  for  the  spiders 
are  also  armed  and  are  bold  and  skillful  fighters.     The  wasp  is 
compelled  to  place  carefully  the  paralyzing  thrust  in  the  midst  of 
a  desperate  conflict. 

856.  Sexual  Dimorphism. — Reference   has   been   made    to 
sexual  dimorphism  (page  353).     This  is  more  general  among 
the  higher  animals.     When  there  is  a  notable  difference  between 


SEXUAL  DIMORPHISM 


423 


the  males  and  females  of  a  species  of  one  of  the  lower  phyla 
the  difference  is  most  frequently  a  difference  of  size.  The 
female  is  usually  larger.  This  is  regarded  as  due  to  the  great 
demands  made  upon  the  female  organism  in  the  development 
of  the  relatively  large  mass  of  egg  substance.  Among  Mammals 
the  male  is  usually  the  larger,  if  there  is  any 
marked  difference  in  size.  The  males  of  the 
fur  seal  and  sea  lion  are  about  four  times  as 
large  as  the  female.  In  such  cases  the  males 
fight  for  the  possession  of  the  females,  and 
consequently  size  and  strength  are  the  deter- 
mining factors  in  the  struggle  for  existence 
among  the  males.  Why  the  size  of  the  male 
parent  should  be  inherited  by  the  male  off- 
spring and  not  by  the  female  is  an  interesting 
problem  which  still  awaits  solution. 

857.  Sexual  Selection. — In  some  species  of 
Birds  also  the  males  are  the  larger,  and  this 
occurs  again  in  those  cases  in  which  the  males 
contend  in  battle  for  the  possession  of  the 
females.  Generally,  however,  the  male  bird  is 
distinguished  from  the  female  by  his  greater  FIG.  260.— 

i  T  .  .  *  •!•,  v   ,        Oueen  of  Termes 

beauty  or  his  superior  ability  as  a  vocalist,  obesus.  Natural 
This  introduces  us  to  a  particular  form  of  size-  (From 
natural  selection,  called  sexual  selection.  At 
the  time  of  mating  the  males  vie  with  each 
other  by  displaying  their  beautiful  plumage  or 
singing  their  best  songs.  This  is  done  in  the 
presence  of  the  female,  and  is  evidently  intended 
to  win  her  favor.  Following  this  courtship  the  birds  mate  in 
pairs  (monogamy),  apparently  according  as  the  appearance  or 
performance  of  the  male  pleases  the  female.  That  is  to  say, 
in  this  case  the  selecting  which  determines  the  male  parentage 
of  the  next  generation  is  done  by  the  female,  and  the  survival 


size . 
F  o  1  s  o  m  after 
Hagen).  It  is 
said  that  in  some 
cases  the  queen 
attains  the  size 
of  thirty  thou- 
sand workers. 


424 


GENERAL  PRINCIPLES 


of  the  male  characters  in  the  next  generation  depends  upon 
the  colors,  plumage  or  song,  which  are  fitted  to  meet  with  the 
approval  of  the  female.  Sexual  selection  also  occurs  in  other 


FIG.  261. — Myrrh,  a  xerophytic  plant  of  the  Arabian  desert.  There  are  few 
leaves  and  the  spiny  branches  offer  protection  against  grazing  animals.  (From 
Sayre.) 

classes  of  animals.  A  particularly  interesting  case  is  found 
among  spiders.  Here  the  males  are  often  much  smaller  than 
the  female,  and  strive  to  gain  her  favor  by  series  of  movements 
which  may  be  called  dancing. 


ADAPTATIONS 


425 


858.  Welfare  of  the  Individual  and  of  the  Species. — The 

welfare  of  a  species  sometimes  makes  demands  which  do  not 
coincide  with  what  is  required  for  the  welfare  of  the  individuals 
of  that  species.  It  is  sometimes  in  the  interest  of  the  species 
that  certain  individuals  should  perish.  It  is  often  better  that 
the  weak  or  inefficient  individuals  should  perish,  and  in  many 


FIG.  262. — A  sandy  beach,  the  home  of  a  small  light-gray  grasshopper.  The 
dark  patches  in  the  background  are  green  yaupon  thickets  where  two  species  of 
green  grasshoppers  are  found.  These  are  remarkably  well  protected  by  their 
coloration  when  in  their  proper  environment.  Coast  of  North  Carolina. 

cases  the  welfare  of  the  species  requires  the  sacrifice  of  the 
most  efficient  individuals.  In  many  instances  the  female 
dies  immediately  after  she  has  properly  deposited  her  brood  of 
eggs.  This  is  not  only  because  her  life  term  has  been  completed 
but  because  of  the  heavy  draught  made  upon  her  organism 
by  the  development  of  the  ova.  Sometimes  the  body  of  the 


426  GENERAL   PRINCIPLES 

female  disintegrates  normally  in  order  to  free  the  contained 
embryos. 

859.  Though  such  cases  are  rather  numerous  they  must 
still  be  regarded  as  exceptional.  Usually  the  best  interests 
of  the  species  are  served  by  that  which  favors  the  individual. 
We  will,  therefore,  inquire  into  that  class  of  adaptation  by 
which  the  individual  profits  more  directly.  Much  of  the 
struggle  for  existence  is  in  reality  a  struggle  between  indi- 
viduals; it  may  be  individuals  of  the  same  or  of  different 


FIG.  263. — A  Juncus  swamp  in  which  is  found  a  large  gray  and  olive-brown 
striped  grasshopper.  The  color  of  the  grasshopper  is  protective  in  such  an 
environment.  Coast  of  North  Carolina. 

species.  Illustrations  of  thi?  principle  are  usually  taken 
chiefly  from  animals,  but  one  from  the  vegetable  kingdom 
may  also  be  introduced  here.  The  difficult  conditions  under 
which  desert  plants  grow  makes  the  number  of  individual 
plants  which  succeed  relatively  small.  Therefore,  the  life 
of  an  individual  plant  is  of  more  value  to  the  species.  It  is, 
therefore,  not  strange  that  such  plants  are  wonderfully  well 
protected.  The  spines  of  the  cacti  render  them  practically 
immune  to  the  attacks  of  animals.  To  a  less  degree  spines 


PROTECTIVE  COLORATION 


427 


are  also  found  on  mescphytic  vegetation  and  are  also  more  or 
less  efficient  as  protection.  Many  plants  are  protected  by 
bitter,  acrid,  poisonous  or  otherwise  disagreeable  juices  which 
protect  them  from  the  attacks  of  many,  if  not  all,  animals. 
When  we  consider  how  devastating  the  attacks  of  insects 


FIG.  264. — Protective  resemblance  of  the  moth,  Catocala  lacrymosa,  to  the 
bark  on  which  it  rests.  A,  With  wings  spread,  as  in  flight;  B,  with  wings  folded 
and  at  rest  on  bark.  (From  Folsom.) 

sometimes  become  to  given  species  of  plants  we  may  realize 
how  useful  such  protective  contrivances  may  be. 

860.  Animal  Coloration. — If  the  color  of  animals  has  no 
general  significance  it  does  have  an  extremely  far-reaching 


428 


GENERAL  PRINCIPLES 


significance  when  we  consider  the  life  of  the  individual.  The 
color  is  often  the  most  remarkable  adaptive  feature  about  it. 
Animals  are  usually  darker  above  than  below.  This  may  be 
due  in  part  to  the  direct  tendency  of  light  to  produce  pigment 
in  the  skin.  But  it  has  also  been  suggested  that  the  dark 
upper  surface  in  the  bright  light  and  the  light  under  surface 


FIG.  265. — Walking-stick  insect  (Diapheromera).     Natural  size.    An  example 
of  protective  resemblance.     (From  Galloway  after  Folsom.) 

in  the  shadow  of  the  body  yield  approximately  equal  lighl 
value  and  tend  to  render  the  animal  inconspicuous  at  a  distance 
86 1.  Animals  generally  are  colored  in  harmony  with  theii 
surroundings.  Polar  animals  are  white;  animals  of  the  deserl 
and  plain  are  gray,  and  those  of  the  forest  are  striped  or  mot- 
tled. Pelagic  marine  animals  are  often  transparent  and  those 
of  the  bottom  are  often  so  much  like  the  bottom  that  it  be- 


PROTECTIVE  COLORATION  429 

comes  difficult  to  distinguish  them  from  their  surroundings, 
even  when  one  knows  precisely  their  location.  Many  animals 
living  on  or  among  green  foliage  are  as  green  as  the  leaves. 
These  agreements  in  color  between  the  animal  and  its  environ- 
ment render  the  animal  difficult  to  see  and,  therefore,  protect 
it  from  its  enemies.  The  same  characteristic  of  the  animal 
enables  it  to  steal  upon  its  prey,  but  in  either  case  the  color  is 
an  advantage  and  may  have  been  developed  through  natural 
selection. 

862.  It  has  been  shown  that  some  color  patterns,  which  at 
first  sight  seem  to  render  the  wearer  conspicuous,   have  in 


FIG.  266. — Protective  resemblance.     A  sea-horse  resembling  a  sea-weed. 
(From  Galloway  after  Eckstein.) 

reality  an  obliterating  effect  when  seen  at  a  distance  under 
natural  surroundings.  Nevertheless,  there  are  many  cases 
in  which  the  color  makes  the  animal  conspicuous.  This  may 
be  illustrated  by  the  many  species  of  birds  in  which  the 
male  is  brilliantly  colored.  An  explanation  for  this  coloring 
has  already  been  given.  But  the  females  of  these  same  species 
are  usually  very  plainly  colored  and  harmonize  well  with  their 
surroundings.  The  female  usually  broods  over  the  eggs,  and 


43° 


GENERAL  PRINCIPLES 


B 


FIG.  267. — The  mimicry  of  Papilio  merope. 

A,  Papilio  merope  (male). 

B,  Papilio  merope,  female;  mimics  Amauris  echeria. 

C,  Papilio  merope,  female;  mimics  Danais  chrysippus. 

D,  Papilio  merope,  female;  mimics  Amauris  niavius. 


ADAPTATIONS  431 

when  sitting  quietly  is  not  readily  seen.  She  is,  therefore, 
probably  less  frequently  molested.  The  male  is  free  to  take 
to  flight  when  discovered  and  pursued.  The  color  of  the  female 
is  explained  on  the  basis  of  ordinary  natural  selection,  while 
that  of  the  male  is  due  to  the  operation  of  sexual  selection. 

863.  Protective  Resemblance. — In  many  cases  the  animal 
resembles  its  environment  in  form  as  well  as  color.     This  is 
called    protective    resemblance.     There    are    insects    which 
resemble    dead    twigs,    rolled   and  broken   dry  leaves,   dead 
leaves  still  on  the  twig,  green  leaves,  seed  pods,  patches  of 
lichens,  etc.     There  are  fishes  which  resemble  sea  weeds  and 
even  a  mammal,  the  sloth,  resembles  a  lichen-covered  knot  on  a 
tree. 

864.  Feigning. — Many  animals  when  threatened  by  enemies 
resort  to  bluff.     They  assume  terrifying  attitudes,  make  a  show 
of  great  size  by  swelling  themselves  or  raising  hair  or  feathers 
on  end,  or  make  disconcerting  noises  like  hissing,  spitting  or 
growling.      Feigning  death  or  "possuming"  is  another  com- 
mon instinctive  method  of  getting  out  of  a  tight  place.     The 
opossum  is  a  well-known  example    and  has  lent  his  name  to 
this   particular   instinct.     Beetles    often   feign  death.     When 
attacked  they  allow  themselves  to  fall  to  the  ground  and  lie 
there  motionless  for  some  time.     They   are  then  difficult  to 
find,  whereas  if  they  attempted    to   run  or  fly   their   move- 


The  three  types  of  female  Papilios,  shown  on  the  left  in  B,  C,  and  D,  belong  to 
the  same  species,  of  which  the  male  is  represented  in  A .  There  is  also  a  type  of 
female  which  resembles  the  male,  and  still  another  form  which  is  not  figured 
here.  There  are  then  five  types  of  females  within  this  species.  Three  of  these, 
beside  the  male,  have  been  reared  from  the  same  brood  of  eggs.  This  species 
is  found  in  Africa  but  a  similar  case  of  polymorphism  is  found  in  India.  The 
origin  of  the  polymorphism  in  this  case  is  apparently  due  to  mimicry.  The 
species  of  Amauris  and  Danais  represented  on  the  right  in  the  figure  are  pro- 
tected, i.e.,  they  are  unpalatable  to  birds,  hence  the  female  Papilios  by  mimicry 
also  secure  immunity  from  the  attacks  of  birds  though  they  are  not  otherwise 
protected.  The  males  are  not  protected,  nor  are  they  mimics,  but  they  are  pro- 
duced in  much  greater  number  than  the  females.  In  A  the  predominant  color 
is  yellow,  in  C  it  is  orange,  while  in  B  and  D  it  is  white,  or  pale  yellow,  and  dark 
gray  to  black.  Xi/2. 


432  GENERAL  PRINCIPLES 

ments  would  make   them   conspicuous  and  invite    a  second 
attack. 

865.  Mimicry. — Many    animals    are    protected   in    various 
degree  by  their  stings,   poison   fangs,   malodorous   secretions 
or  unpalatable  taste.     These  are  naturally  avoided  by  species 
which  would  otherwise  prey  upon  them.     But  this  means  that 
the  preying  species  must  be  able  to  distinguish  between  the 
palatable  and  unpalatable  prey.     The  unpalatable  forms  are 
often  conspicuously  marked,  as  if  to  advertise  the  fact,  and 
thus  prevent  an  attack  which  might   be  disastrous  to  both 
pursuer  and  pursued.     Coloring  which  is  regarded  as  serving 
such  an  end  is  termed  warning  coloration.     It  is  especially 
common  among  insects  and  protects  them  from  birds. 

866.  Where  species  occur  which  are  protected  and  warn- 
ingly  colored  there  are  also  often  other  species  which  are  not 
protected  by  sting  or  unpalatable,  and  yet  are  very  like  the 
protected  species  in  form  and  color.     This  is  called  "  mimicry," 
because  the  one  form  is  supposed  to  have  acquired  a  resem- 
blance to  the  other  for  the  purpose  of  protection.     If  a  bird 
recognizes  a  certain  form  and  color  pattern  as  belonging  to  an 
undesirable  insect  another  insect  resembling  the  first  would  be 
spared  an  attack  if  the  bird  failed  to  discriminate.     Upon  this 
ground  mimicry  is  explained  by  natural  selection.     A  fact  of 
almost  conclusive  significance  is  that  the  mimic  and  the  model 
are  always  found  associated  in  the  same  regions.     Mimicry 
is  very  common  among  butterflies,  but  many  cases  are  known 
in  which  bees,  bumbleebees  and  wasps  serve  as  models  and  are 
mimicked  by  flies,  beetles  and  butterflies.     Cases  in  which 
poisonous  snakes  are  mimicked  by  harmless  ones  are  also  known. 

867.  Mimicry  also  occurs  between  two  species  which  are 
both  protected.     This  demands  a  different  explanation  from 
the  preceding  case.     Birds  learn  that  protected  species  are 
unpalatable  only  by  experience,  and  in  getting  this  knowledge, 
one  or  more  butterflies  of  the  protected  species  are  injured  or 


ADAPTATIONS  433 

destroyed.  Therefore,  each  species  profits  by  the  sacrifice 
the  other  makes  in  the  education  of  the  bird.  In  the  case  of 
the  unprotected  mimic,  however,  the  burden  of  education  rests 
wholly  on  the  protected  model. 

868.  Color  Changes. — The  color  of  animals  is  in  the  main 
inherited,  but  it  may  often  be  more  or  less  modified  in  the 
individual  by  the  environment.     Animals  kept  in  darkness 
tend  to  become  paler,  and  if  the  young  flat  fishes  are  illuminated 
from  below  there  is  a  tendency  for  the  underside  to  develop 
pigment  where  normally  there  is  none.     The  color  of  cater- 
pillars may  be  determined  somewhat  by  the  color  of  the  sur- 
rounding objects.     If  they  are  surrounded  by  dark  objects 
they  tend  to  become  darker  also.     Other  animals,  like  many 
fishes,  frogs,  tree  toads,  lizards  and  cuttlefishes,  change  color 
rapidly  and  repeatedly.     The  color  of  a  cuttlefish  changes  in  a 
flash.     This  is  due  to  the  action  of  contractile  pigment  cells. 
When  the  cells  expand  the  animal  takes  on  the  color  of  the 
chromatophores,  dark  brown  or  orange  or  a  combination  of 
these  colors,  as  the  different  cells  are  stimulated.     The  action 
is  controlled  through  the  nervous  system,  but  it  is  not  clear 
what  service  it  plays  in  the  animal  economy.     The  chameleon 
and  other  lizards,  as  well  as  many  frogs  and  fishes,  change 
color  more  slowly,  but  by  a  similar  mechanism.     In  these 
cases  the  color  assumed  is  determined  by  the  surroundings. 
If  the  animal  is  blind  the  changes  do  not  occur,  and  it  is  known 
that  the  stimulus  is  received  through  the  eye  and  transmitted 
by    the    nervous    system    without,    however,    any   voluntary 
control  by  the  animal.     In  these  cases  the  colors  assumed  are 
protective. 

869.  Luminescence. — There  are  two  important  physiological 
processes  which  are  practically  wanting  in  all  animals  above 
fishes,  but  are  quite  common  among  lower  forms.     These  are 
the  light-producing  organs  and  organs  for  generating  electricity. 
The  production  of  light  or  phosphorescence  occurs  among  all 

28 


434  GENERAL  PRINCIPLES 

classes  of  animals  from  the  Protozoa  to  Fishes,  and  also  among 
fungi.  Decaying  vegetable  and  animal  matter  is  often  lumi- 
nous as  a  'result  of  the  action  of  bacteria.  The  mycelium  of 
other  fungi  also  yields  light  at  times.  In  these  cases  the  light 
is  apparently  a  by-product,  which  is  not  to  be  regarded  as 
having  any  adaptive  function. 

870.  The    light-producing    power    is     especially     common 
among    marine    invertebrates   and  Fishes.     Phosphorescence 
occurs  among  the  Protozoa,  Coelenterates,  Worms,  the  smaller 
Crustaceae,  Bryozoa,  Rotifers,  free-swimming  Ascidians,  Fishes 
and  Insects.     In  some  cases  the  light  is  produced  at  certain 
points  within  the  protoplasm  of  the  cell.     In  others  the  slime 
secreted  by  glands  in  the  skin  is  luminous.     And  again  there 
are  special  organs  which  may  be  simple  or  complex  in  structure, 
but  which  bear  evidence  of  having  been  developed  from  groups 
of  glands,  though  there  is  no  duct  and  no  external  secretion. 
The  more  highly  developed  photogenic  organs  have  a  structure 
resembling  that  of  an  eye  with  a  layer  of  pigment  at  the  back 
and  a  lens  in  front.     In  the  lower  animals  the  light  is  only 
emitted  when  the  animal  is  stimulated,  and  the  purpose  of  it  is 
unknown.     In  the  higher  forms,  however,  the  organ  is  well 
supplied  with  nerve  elements  and  is  under  voluntary  nervous 
control. 

871.  Among   Fishes   the   photogenic   organs   are   especially 
common  and  well  developed  in  deep  sea  species.     Some  "  an- 
glers" carry  a  lantern  at  the  tip  of  the  long  anterior  dorsal 
spine.     This  lantern  is  suspended  directly  above  the  mouth 
of  the  animal  and  is  regarded  as  a  bait  by  which  the  angler 
attracts  his  prey.     In  other  cases  the  photogenic  organs  may 
occur  on  almost  any  part  of  the  body. 

872.  The  common  "fire  fly"  is  a  beetle  belonging  to  the 
family,    "Lampyridae,"    which    contains    many    luminescent 
species.     The  eggs,  larvae,  male  and  female,  may  all  be  lumi- 
nescent.    In  some  species  the  female  is  wingless,  but  has  an 


ADAPTATIONS  435 

exceptionally  brilliant  light,  which  may,  therefore,  enable  the 
winged  male  to  find  her.  In  many  cases  the  eyes  of  these 
nocturnal  insects  are  unusually  well  developed,  as  is  also  the 
case  with  the  deep  sea  fishes,  a  fact  that  renders  probable  the 
view  that  the  luminescent  organs  are  a  means  by  which  the 
sexes  find  each  other.  The  luminescence  of  the  eggs  and  larvae 
are  not  understood.  Photogenic  organs  are  common  in  other 
families  of  beetles,  and  also  occur  among  flies. 

873.  Oxygen  is  said  to  be  necessary  to  the  action  of  photo- 
genic organs,  but  no  appreciable  heat  is  generated. 

874.  Electrical  Organs. — Organs  for  generating  electricity 
are  developed  in  a  number  of  Fishes.     The  electric  eel  (Gym- 
notus)  of  the  Amazon  and  Orinoco  rivers,  the  electric  catfish 
(Malapterurus)  of  tropical  Africa,  and  the  electric  rays  (Tor- 
pedo)   of  the  warmer  seas  are  all  capable  of  producing  an 
electrical    discharge    sufficient    to   stun  large   animals.     The 
electric  organ  of  the  Gymnotus  is  a  modified  muscle  of  the 
ventral  side  of  the  long  tail.     In  Torpedo  the  organ  is  also  of 
modified  muscles,  but  of  the  head  region.     In  Malapterurus  the 
glands  of  the  skin  have  been  the  starting  point  from  which 
the    electric    organ   developed.     In   neither    case,  of  course, 
does  the  fully  developed  electric  organ  bear  any  resemblance 
to     muscle    or    gland.     The    nerves    supplying    the    electric 
organs    are    developed    to    an  extraordinary  degree  and  the 
electric  discharge  is  under  voluntary    control.     These    organs 
are    doubtless    organs    of   offense  and  defense. 

875.  Instinct. — When  the  Pronuba  moth  deposits  her  egg  in  the 
pistil  of  the  Yucca  and  then  stuffs  pollen  in  the  stigma  she  is  not 
aware  of  the  end  secured  by  her  acts.     She  does  not  know  that 
pollination  will  cause  the  ovules  to  develop.     She  does  not  know 
that  there  are  ovules.     She  cannot  even  know  that  she  has  de- 
posited an  egg.     She  never  sees  the  eggs;  she  never  sees  her  off- 
spring.    The  whole  performance  is  to  her  without  meaning 
and  is  enacted  in  obedience  to  an  internal  impulse  originat- 


436  GENERAL  PRINCIPLES 

ing  in  physiological  processes  connected  with  the  organs  of 
reproduction. 

876.  Practically  the  same  may  be  said  of  the  actions  of  the  mud 
dauber  when  building  her  mud  nest  and  filling  it  with  embalmed 
spiders.  Another  example  may  be  described.  There  is  a  fam- 
ily of  beetles  (Cantharidae)  which  are  parasitic  on  other  insects 
during  the  larval  stages.  In  some  cases  (Sitaris)  the  eggs  of  the 
beetle  are  deposited  on  the  ground  in  the  vicinity  of  a  bumble- 
bee's nest.  A  small  active  larva  hatches  from  these  eggs  and 
this  larva  reaches  the  nest  of  the  bumblebee  in  a  very  peculiar 
way.  It  does  not  seek  the  opening  of  the  nest  and  thus  make 
its  way  in,  but  waits  until  some  living  object  like  the  bumblebee 
chances  to  come  within  reach,  when  it  attaches  itself  to  the  legs 
or  hairs  of  the  body  and  is  thus  carried  into  the  nest.  The 
bumblebee  builds  a  large  cell  of  wax.  This  it  fills  with  honey, 
and  then  deposits  an  egg  on  the  honey  and  seals  up  the  cell. 
At  the  moment  when  the  egg  is  deposited  the  beetle  larva 
attaches  itself  to  the  egg  and  is  thus  sealed  up  in  the  cell  with 
the  egg  and  honey.  It  first  devours  the  egg,  which  requires  al- 
most eight  days'  time.  Then  it  undergoes  a  metamorphosis, 
after  which  it  is  adapted  for  feeding  upon  honey,  which  it  could 
not  do  before.  After  about  forty  days'  feeding  on  honey  the 
supply  is  exhausted  and  the  larva  undergoes  a  second  meta- 
morphosis. This  is  followed  by  several  more  metamorphoses, 
after  which  the  adult  beetle  (blister  beetle)  appears.  The 
notable  thing  about  this  life  history  is  the  means  adopted  by  the 
minute  larvae  for  reaching  the  nest  of  the  bee.  They  will  often 
attach  themselves  to  other  living  objects,  such  as  other  insects 
or  even  a  camel's  hair  brush.  When  they  do  this  they  perish, 
because  they  fail  to  reach  the  condition  necessary  for  their 
future  development.  These  larvae  are  not  taught  what  they 
have  to  do  to  succeed;  they  cannot  profit  by  the  observation  of 
others,  and  they  cannot  learn  from  experience.  They  are 
somehow  impelled  to  attach  themselves  to  other  insects.  If 


INSTINCT  AND   INTELLIGENCE  437 

they  are  fortunate  enough  to  attach  themselves  to  the  bee  they 
succeed.  Otherwise  they  perish.  The  female  blister  beetle  is 
very  prolific  and  deposits  many  thousands  of  eggs.  Hence  it  is 
only  necessary  that  a  few  of  the  thousands  of  larvae  should 
succeed. 

877.  Actions  like  those  of  the  Pronuba,  Sphex  or  blister  beetle 
larva  are  called  instinctive.     They  are  not  prompted  by  a  kind 
of  intelligence.     Nor  are  they  in  any  sense  akin  to  intelligence, 
though  among  the  higher  animals  it  is  often  difficult  to  say 
whether  an  act  is  prompted  by  instinct  or  intelligence. 

878.  If  a  moth  habitually  rests  on  surfaces  which  it  resembles  it 
does  so  instinctively,  not  because  it  has  an  intelligent  compre- 
hension that  it  is  thereby  protected.     When  a  caterpillar  spins 
a  cocoon  it  does  so  instinctively  and  not  with  the  forethought  of 
providing  protection.     When  a  young  bird  builds  a  nest  it  is 
impelled  thereto  by  instinct,  and  the  form  and  manner  of  build- 
ing the  nest  are  also  determined  by  instinct.     In  the  latter  case 
more  or  less  evidence  of  intelligence  may  be  discernible,  but  the 
process  as  a  whole  is  instinctive. 

879.  An  instinct  is  a  kind  of  adaptation  and  subject  to  the  laws 
of  heredity.     Instinctive  actions  may,  therefore,  be  developed 
under  natural  selection,  just  like  other  adaptive  characters  of 
an  organism. 

880.  Intelligence. — Intelligence  is  found  only  among  the  most 
highly  organized  animals  because  it  is  dependent  upon  an 
efficient  set  of  sense  organs  which  yield  accurate  information 
concerning  the  environment,  a  flexible  response  mechanism 
which    may    react    in    multitudinous    ways    to    the  infinite 
variation  in  the  conditions  of  existence,  and  an  organ  of  con- 
trol, the  brain.     In  practically  all  Birds  and  Mammals  the  first 
two  of  these  conditions  ©^intelligence  are  well  met,  and  yet  there 
is  a  vast  difference  in  intelligence  within  these  classes.     This  is 
due  to  the  difference  in  brain  structure.     The  brain  of  the  lowest 
Vertebrates  is  an  inconceivably  complex  organ,  and  in  the  higher 


438  GENERAL   PRINCIPLES 

forms  it  is  vastly  more  so.  This  organ  enables  the  individual 
animal  to  profit  by  experience.  A  sensation  or  an  experience 
of  any  kind  to  which  the  organism  has  once  been  subjected  is  in 
some  way  registered  in  the  brain  (memory)  and  through  it  the 
future  responses  are  modified.  The  constant  stream  of  highly 
complex  stimuli  which  pour  in  upon  the  organsim  from  the  en- 
vironment are  sifted  and  analyzed  in  some  way  by  the  brain, 
and  the  appropriate  responses  determined  (reason,  judgment), 
and  the  proper  motor  stimuli  sent  out  to  the  organs  of  response 
(will) .  Animals  guided  by  instinct  inherit  a  few  sets  of  more  or 
less  complex  responses,  which  are  set  in  motion  by  corresponding 
sets  of  stimuli,  and  these  responses  are  little  if  at  all  modified. 
Responses  prompted  by  intelligence  are  more  variable  as  de- 
termined by  variable  external  conditions,  and  the  individual 
exercises  an  adaptive  control.  The  brain  might  be  called  an 
organ  of  adaptation,  for  the  degree  in  which  the  individual  can 
adapt  itself  to  its  environment  is  a  measure  of  its  intelligence. 


INDEX-GLOSSARY. 


Acarina,  571 

Accessory  buds.  57 

Accipitres,  650 

Achatinella,  distribution  of,  828 

Achene,  156 

Acicula,  a  needle-shaped  structure,  346 

Acrania,  (Gk.  having  no  skull),  624 

Actinomma,  126 

Adaptations,  829 

regarding  food,  844 
light  838 

temperature,  839,  840 
water,  837 

Adelochorda,  617 

Adventitious  buds,  57 

^Ecidiospores,  749 

yEpyornithes,  650 

African  fauna,  826 

Agglutinin,  777 

Aggregate  fruit,  164 

Air  sacs,  647 

Albatross,  650 

Albumen,  the  white  of  an  egg,  677 

Aleurone,  19,  95,  672 

Alexin,  (Gk.  to  ward  off),  776 

Alga,  pi.  algae  (L.  a  sea  weed). 

Algae,  177,  178 

blue-green,  232 

Allantois,  one  of  the  fcetal  membranes 
of  reptiles,  birds  and  mam- 
mals, 174,  641,  651 

Alligator,  644 

Alternation  of  generations,  525,  741 

Altitude  and  plants,  204 

Ambulacral,  pertaining  to  the  system 
of  tube  feet  of  an  echino- 
derm. 

Ambulacral  system,  550 

Amiatus,  635 

Amnion,  one  of  the  fcetal  membranes 
of  reptiles,  birds  and  mam- 
mals, 641,  651 

Amoeba,  68,  178,  303 

chemical  sense  of,  353 
digestion  in,  452,  453,  464 


Amceba,  reproduction  in,  495,  496 

respiration  in,  476 

response  in,  407 

sense  of  sight,  355 
of  touch,  354 

temperature  sense,  356 
Amcebina,  510 
Amphibia,  350,  637 

structure  of,  637 
Amphineura,  593 
Amphioxus,    syn.    for   Branchiostoma 

(lancelet). 

Amphipoda,  561,  563 
Amphitrite,  139 

Amphiuma,  the  "congo  snake." 
Ampulla,  550 
Amylolytic,    converting    starch    into 

sugar,  461 

Amylolytic  ferments,  463 
Amyloplast — a    starch    forming    cor- 
puscle, 93,  672 
Amylopsin,  462 
Andreaceae,  277 
Andrcecium,  123,  128 
Anemophilous,  pollinated  by  the  wind, 

139,  140 

Angiosperms,  171,  298 
Angler  fish,  844 
Animal  coloration,  860 
Anisopoda,  561 

Annelida  (L.  annellus — a  ring)  worms 
with  a  segmented,  ringed 
body,  542 

Annelids,  circulatory  system  of,  469, 
470 

locomotion  of,  410,  411 

nervous  system  of,  438 

reproductive  system  of,  501,  502 

trochophore  larva  of,  501 
Anomostraca,  561 
Anomura,  561,  562 
Anopheles,  768 
Ant  bear,  66 1 
Ant  eater,  spiny,  176 
Antedon,  144 
Antelope,  666 


excretion  in,  489 
Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 

439 


44o 


INDEX — GLOSSARY. 


Antenna,  one  of  the  first  or  second 
pair  of  jointed  appendages, 
"feelers,"  on  the  head  of  an 
Arthropod. 

Antennae,  of  moth,  78 

Antennule,  one  of  the  first  pair  of 
jointed  appendages  on  the 
head  of  a  Crustacean. 

Anterior,  311 

Anther,  123 

Antheridium,  264 

Anthocerotaceae,  270 

Anthozoa,  529 

Antibodies,  777 

Antimere,  335 

Antitoxin,  776,  777 

Antlers,  348 

Ant  lion,  585 

Ants,  590 

polymorphism  in,  740 

Anura,  640 

Anvil,  403 

Apanteles,  236 

Apes,  652,  668 

Aphids,  227,  590 

Apical  cell,  710 

Aplanospores,  non-motile  spores,  247 

Apothecium — the  concave  fruiting  sur- 
face of  a  fungus,  258 

Appendages,  of  Arthropods,  412 
of  Vertebrates,  170,  413,  792 

Apteryges,  650 

Apterygogenea,  577 

Apteryx,  175,  650 

Aquatic  plants,  184-191 

Aqueous  humor,  382 

Arachnoidea,  566 

Araneida,  569 

Archaeop  teryx,  247,  806 

Archegoniates,  those  cryptogams 
which  bear  archegonia,  viz., 
Bryophyta  and  Pterido- 
phyta,  266 

Archegonium,  264 

Archenteron,  the  primitive  intestine, 

7U 

Arethusa,  848 

Aristotle's  lantern,  555 

Armadillo,  421,  652,  661 

Artemia,  687 

Arthostraca,  561,  563 

Arthropoda  (Gk.  jointed  foot),  557 

Arthropods,  appendages  of,  412 
chemical  sense  of,  370 
circulatory  system  of,  471 
digestion  in,  458,  459,  467 


Arthropods,  eyes  of,  378 

exoskeleton  of,  417 

glands  of,  350 

growth  in,  731 

locomotion  of,  412 

nervous  system  of,  443 

segmentation  of,  336 

sense  organs  of,  361 
Artiodactyla,  666 
Ascaris,  687,  695,  758 
Aschelminthes,  538 
Ascidians,  621 

structure  of,  621 

symmetry  of,  321 
Asclepias,  851 
Ascomycetes,  256 
Ascus  (Gk.  a  bag),  256 
Ash,  64 

Aspidobranchia,  600,  601 
Assimilation — the   process   of   trans- 
forming food  into  the  living 
substance,  6,  486 
Association  fibres,  439-441, 
Asteroidea,  552 
Asymmetry,  318 
Atrium,  621 

Attraction   sphere — a  rounded  mass 
of  the  cytoplasm  which  en- 
closes the  centrosome,  685 
Auditory  organ,  396 

meatus,  401 
Auricle,  401 
Australian  fauna,  825 
Autobasidiomycetes,  255 
Autogamy — self  fertilization,  144 
Aves,  646 

structure  of,  646 
Axil,  56 

Axillary  bud,  56 
Axis  of  locomotion,  310 

Babesia,  771 
Bacillus,  227 

aceti,  229 

anthracis,  231 

coli  communis,  231 

diphtherise,  231 

pneumonias,  231 

tetani,  231 

tuberculosis,  231 

typhi,  ^231 

vulgaris,  230 
Bacteria,  58-61,  180,  217,  227-231, 

673,  778 

as  parasites,  772,  773 
nucleus  of,  673 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


441 


Bacterium,  227 

Balancers,  587 

Balanoglossus,  syn.  for  Dolichoglossus 

Baleen  whale,  665,  796 

feeding  of,  844 
Ball  and  socket  joint,  413 
Barnacles,  560,  739 

symmetry  of,  321 
Basidiomycetes,  252 
Basidium,  252 
Basilar  membrane,  398 
Basket  fish,  147,  554  ^ 
Bast,  the  fibrous  portion  of  the  back, 

74 

Bat,  652 
Beak,  347 
Bears,  652,  663 
Beavers,  652,  660 
Bed  bugs,  591 
Bees,  590 

Beggiatoa  alba,  229 
Berry,  153 

Bilateral  symmetry,  315 
Bile  pigments,  483 
Bimana,  669 
Biology,  i 
Birds  (see  Aves). 
Birds  and  reptiles,  792 
Birds,  development  of,  806 

glands  of,  350 

migration  of,  843 

reptilian  characters  of,  649 
Blackbirds,  650 
Black  mold,  250 
Blastula,  196,  197,  712 

of  lancelet,  712 
Blood,  origin  of,  721 

-vessels,  origin  of,  721 
Blue  crab,  ecdysis  of,  76 
Blue  flag,  pollination  of,  253,  254 
Blue-green  algae,  nucleus  of,  673 
Blue  molds,  257 
Body  cavity,  469 

fluid,  469 

Bojanus,  organ  of,  607 
Bone,  95,  426 
Bones,  origin  of,  721 
Bony  labyrinth,  399 
Book  gills,  565 

lungs,  567,  568,  569 
Bot  fly,  761 
Brachiopoda,  549 
Brachyura,  561,  562 
Bract — the     leaf,    usually    modified, 
which  subtends  a  floral  shoot. 
Brain,  444,  580 


Brain,  of  annelids,  438 
of  crayfish,  100 
of  shark,  102 
Branchioganoidea,  632 
Branchiata,  558 
Branchiura,  559 
Brand  spores,  253 
Brittle  star,  146,  554     . 
Bryinae,  279 
Bryophyllum,  34 

Bryophyta — Bryophytes,  176,  264 
Bryozoa,  548 
Budding,  330,  671 
in  hydra,  499 
multiplication  by,  169 
Buffalo,  666 
Bugs,  763 
Bulbus  arteriosus,  a  muscular  chamber 

of  the  heart,  in  front  of  the 

ventricle,  628 
Bull-bat,  650 
Butterflies,  586 

polymorphism  in,  743 
Byssus,  silky  fibres  spun  from  a  gland 

in  the  foot  of  certain  Lamelli- 

branchs,  610 

C — symbol  for  carbon. 

Cactus,  24 

Caddice  worm  nets,  251,  252 
shelters,  73,  74 

Caecum — ccecum — cecum, — a  pouch 
or  sac. 

Calyptra,  part  of  the  archegonium 
which  remains  attached  to 
the  spore  capsule,  277 

Calyx,  122,  131 

Cambium,  75 

Camel,  666 

Cameleon,  645 

Campodea,  159 

Canaliculi,  of  bone,  426 

Canal  system  of  sponge,  521 

Canidae,  663 

Capillary,  small  blood-vessels  having 
walls  composed  of  a  single 
layer  of  thin  cells. 

Capitulum,  119 

Caprella,  150 

Capsule,  163 

Carapace,  a  hard  case  or  shell,  643 

Carbohydrates — organic  compounds  of 
carbon  hydrogen  and  oxygen, 
with  the  hydrogen  and  oxy- 
gen in  the  proportions  of 
H20,  94 


Numbers 


to  paragraphs;  Black  Face  numbers  refer  to  figures. 


442 


INDEX — GLOSSARY. 


Carbohydrates,  absorption  of,  472 
Carbon    dioxide,     formed    in    body, 
488,  489 

used  by  plant,  85 
Carbon  of  plant,  source  of,  89 
Carinatae,  650 
Carnivora,  652,  662 
Carnivorous  — :  flesh-eating. 

plants,  218 
Carp,  636 
Carpels — the  leaves  modified  to  form 

the  pistil,  28,  125 
Carpogonium,  248 
Carpospores,  248 
Cartilage,  49,  425 

origin  of,  721 
Caryopsis,  157 
Cassia,  42,  43 
Cassowary,  650 
Catarrhina,  668 
Catbird,  650 
Caterpillar,  723 
Catfish,  636 
Catkin,  119 
Cats,  663 
Cattle  family,  666 
Cave  fauna,  838 
Cedar  apple,  750 
Cell,  2,  16,  179 

colonies,  704 

division,  683 

masses,  705 

membrane,  672 

regeneration  of,  680 

wall,  622 
Cellulose,  96,  619 
Central  nervous  system,  325 

of  annelids,  438 

origin  of,  715 
Centrosome,  677,  683,  689 
Centipede,  576 
Cephalization,  311 
Cephalomya,  762 
Cephalopoda — Cephalopods,  611,  616 

structure  of,  611 
Cercaria,  754 
Cermatia,  576 

Cestoda— Cestodes,  537,  755 
Cetacea,  652,  665 
Chaetopoda,  543 
Chaetopterus,  tube  of,  72 
Characeae,  244 
Chela,  a  pincer-like  claw. 
Chelicerae,  the  first  pair  of  appendages 
of  certain  Arthropods,  567 


Chemical  sense,  353 

of  arthropods,  370 

of  crayfish,  369 

of  earthworm,  368 

of  hydra,  367 

of  amoeba,  353 
Chilopoda,  576 
Chimpanzee,  668 
Chinch  bugs,  591,  763 
Chiroptera,  652,  659 
Chiton,  341,  593 
Chlorophyceae,  238 
Chlorophyll,  82 
Chloroplasts,  82 

position  in  cell,  206 
C6HioO6— starch,  87 
CeH^Oe — sugar,  90 
Chondroganoidea,  633 
Choroid  layer,  379 
Chromatin,  674 

Chromatophore — a     cell    containing 
pigment  or,  a  protoplasmic 
granule  containing  pigment. 
Chromoplast — a   protoplasmic    gran- 
ule (plastid)  containing  pig- 
ment, 672 
Chromosome,  683 
Chromosomes,  division  of,  684,  695 

individuality  of,  703 

number  of,  687,  695 
Cicada,  396,  591,  763 
Ciconiae,  650 
Cilium(pl.  cilia),  minute  thread-like, 

vibratile  appendages. 
Ciliary  body,  381 
Ciliata,  514 
Circulation,  327,  468- 
Circulatory  system  of  annelids,  469, 
470 

of  arthropods,  471 

of  crayfish,  no 

of  man,  114 

of  Nereis,  112,  113 

of  vertebrates,  472 
Circumvallate  papillae,  371 
Cirratulus,  140 
Cirripedia,  560 
Civet  cat,  663 
Clam,  166 
Clamatores,  650 
Classes  of  animals,  507-669 

of  plants,  170-181,  224-301 
Claws,  347 
Cleavage,  702- 

typesof,  711,  718 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


443 


Cleistogamic — flowers  which  do  not 
open  and  which  are  self 
fertilized,  145 

Climate  and  vegetation,  223 

Climbing  plants,  212 

Cloaca,  the  chamber  into  which  the 
intestine,  ureters,  and  gono- 
ducts,  all  open,  556,  637 

Closteridium  butyricum,  229 

Club  moss — Lycopodium. 

Clypeaster,  65,  148 

Cnidaria,  524 

Cnidoblasts,  524 

COz — symbol  for  carbon  dioxide,  66 

Coal,  219,  221 

Coccus,  227 

Coccygomorphae,  650 

Cochlea,  398 

Cockroaches,  582 

Cocoon,  586,  590 

Ccelenterates,  518, 

chemical  "sense  of,  367 
digestion  in,  454,  465 
nerve  cells  of,  435 
sense  organs  of,  359 

Ccelomata,  532 

Coleoptera,  589 

Collar,  of  Dolichoglossus,  618 

Coloration,  protective,  86 1,  862 

Color  of  animals,  308,  860 

Color  changes,  868 

Columbae,  650 

Columella,  270,  403 

Commensalism,  745 

Composition  of  plants,  63-65 

Compound  eyes,  378,  580 

Conchiolin,  605 

Conchifera,  594 

Condor,  650 

Condylarthra,  666 

Cones  of  retina,  380 

Coney,  666 

Congo  snake,  243  1 

Conidium — spores  formed  in  fungi 
by  budding,  249 

Coniferae,  296 

Conjugate,  243 

Conjugation,  182,  186,  692- 

Conjugation  in  protozoa,  498,  699 

Connective  tissue,  96,  325 

Connective  tissue,  origin  of,  721 

Conus  arteriosus,  a  chamber  of  the 
heart  lying  in  front  of  the 
ventricle  and  containing  sev- 
eral valves,  628 

Copelata,  620 

Numbers  refer  to  paragraphs;  Black  Face 


Copepoda,  559 

Coral,  75,  135-138,  349,   529 

Cordyceps,  229 

Cork,  101 

Cork  cambium  101 

Corm,  in 

Cornea,  379 

Corolla,  122,  132 

Corrodentia,  583,  740 

Corydalis,  585 

Corymb,  119 

Cotyledons,  294 

Crabs,  561 

Crab,  lung  of,  563 

terrestrial,  563 
Cranes,  650 
Cranial  nerves,  445 
Crayfish,  561,  563 

chemical  sense  of,  369 

development  of,  504,  505 

digestion  in,  467 

green  gland  of,  491 

integument  of,  341 

locomotion  of,  412 

reproduction  in,  503,  504 

respiration  in,  477 

statocyst  of,  390 
Cricket,  396,  582 
Crinoids    (sea    lilies),    an    order    of 

Pelmatozoa. 
Crocodile,  644 
Crop,  579 

Cross  fertilization,  138,  142 
Cross-striped  muscle  fibre,  414 
Crows,  650 
Crustacea,  558 
Cryptogams,  174-181,  220,  292 

reproduction  in,  181 
Ctenidia,  comb-like  gills,  593,  595 
Ctenobranchia,  600,  601 
Ctenoid  (of  a  fish  scale) — with  a  spiny 

edge,  636 
Ctenophora,  531 
Cuckoo,  650 
Cumacea,  561 

Cuscuta  (dodder) — a  genus  of  para- 
sitic flowering  plants. 
Cuticula,  of  hydroids,  339 

of  w6rm,  340 

Cutinized  cell  walls,  97,  98 
Cuttle  fish,  611 
Cyanophyceae,  232 
Cycadinae,  294 
Cycloganoidea,  635 
Cycloid  (of  a  fish  scale) — with  smooth 

rounded  edge,  635 
numbers  refer  to  figures. 


444 


INDEX — GLOSSARY. 


Cyclosporeae,  247 

Cyclostomata — Cyclostomes,  627 

structure  of,  627 
Cyme,  118 

Cymose  inflorescence,  25 
Cynips,  764 
Cyprepedium,  850 
Cypselomorphae,  650 
Cysticercus,  755 
Cystoflagellata,  512 
Cytoplasm,  672 

division  of,  685 

function  of,  680,  682 

Daddy-long-legs,  570 
Damsel  flies,  584 
Darning  needle,  582 
Dasypeltis,  844 
Day  flies,  584 
Decapoda,  561,  616 
Decay,  670 

Deciduous,   falling   off    (especially  of 
leaves). 

plants,  197-198 
Deer  family,  666 
Degeneration,  588 
Dehisce  (of  a  fruit) — to  open. 
Dentine,  the  bony  substance  of  teeth 
lying  beneath  the  enamel,  348 
Dermis,  70 

of  mammal,  343 

origin  of,  721 
Desmidiaceae,  235 
Development  of  crayfish,  504,  505 

of  insect,  505 

of  vertebrate,  506 

progressive,  738 

regressive,  738 

types  of,  707 
Devil  fish,  167 
Devil  ray,  169 

Devil's  horse — praying  mantis,  582 
Diadelphous,  133 

Diastatic  (of  a  ferment) — having  the 
power   of   converting   starch 
into  sugar. 
Diatomae,  233 
Diatomes,  growth  of,  727 
Dibranchiata,  616 
Dichogamy,  31,  142 
Dicotyledons,  171,  172,  300 

development  of,  709 
Difflugia,  i 
Differentiation,  323,  725 

of  tissues,  22,  96 
Digestion,  326,  452- 


Digestion  by  bacteria,  453 

in  amoeba,  452,  453,  464 

in  arthropods,  458,  459,  467 

in  ccelenterates,  454,  465 

in  crayfish,  467 

in  flatworms,  454 

in  hydra,  454 

in  sponges,  454 

in  worms,  455-457,  466 
Digestive  ferments,  463 

glands  of  vertebrates,  461 

tract  of  crayfish,  no 
of  grasshopper,  161 
of  man,  in 

of  Nereis,  104-107,  109 
of  vertebrates,  460 
Digitigrade    (of  certain  mammals) — 

standing  on  the  toes,  663 
Dimorphic — two  types  of  form  in  the 

same  species,  143 
Dimorphism,  225,  226 

seasonal,  742 

sexual,  739,  856 
Dinoflagellata,  512 
Dinornis,  650 
Dinornithes,  650 
Dioecious,  128 
Dionaea,  50-52 
Diphycercal,  631,  632 
Diplopoda,  575 
Dipnoi,  631 

Diprotodontia,  652,  656 
Diptera,  587 
Discomycetes,  258 
Dispersal  larvae,  831 
Divers,  650 
Division  of  labor,  725 
Dodder,  230,  751 
Dodo,  650 
Dogs,  652 
Dogwood,  33 
Dolphin,  652,  665 
Dolichoglossus,  618 

gill  slits  of,  618 

nervous  system  of,  618 

notochord  of,  618 

structure  of,  618 
Domesticated  animals,  820 

plants,  820 
Dominant    characters    (in    heredity), 

783 

Doodle  bug,  585 
Dorsal,  312 
Dove,  650 
Dragon  flies,  584 
Dromedary,  666 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures, 


INDEX — GLOSSARY. 


445 


Drosera,  49 
Drupe,  154 
Ducks,  650 
Duck  bill,  177,  652 

mole — duck  bill. 
Dugong,  652,  667 

Eagles,  650 

Ear,  84 

Ear  drum,  401-403 

Ear  of  insects,  397 

Ear  of  vertebrates,  398 

Ear  sacs,  387 

Ear  stone,  392 

Earthworm,  chemical  sense  of,  368 

locomotion  of,  411 

respiration  in,  476 
Earthworms,  305,  543 
Earwigs,  582 

Ecdysis,    molting,    or   shedding,    the 
superficial  layers  of  the  in- 
tegument,  214,    731 
Echidna,  syn.  for  Tachyglossus,  the 

spiny  ant  eater. 
Echinodermata,  550 
Echinoderm  larva,  64 
Echinoidea,  555 

Ecology. — The   science  of   the   rela- 
tion  of  organisms   to   each 
other  and  to  their  inanimate 
environment,  182-223 
Ectoderm,  713 

of  hydra,  339 
Ectoplasm,  672 
Edentata  nomarthra,  652,  66 1 

xenarthra,  652,  661 
Eels,  636 

Effectors,  439-441 
Efts,  639 
Egg,  671 

apparatus,  the  egg  nucleus  and 
two  other  nuclei,  synergids, 
which  lie  together  at  one  end 
of  the  embryo  sac,  299 

maturation  of,  696 

membrane,  702 

of  bird,  647 

pronucleus,  698,  699 

size  of,  697 

yolk  of,  711 
Eimeria,  127 
Elaters,  267,  287 
Electrical  organs,  874 
Electric  eel,  636,  874 

light  bug,  591 
Elephants,  666 


Elytra,  589 
Embryo,  43,  44 

human,  249 

of  dicotyledon,  189,  709 

of  fern,  188,  709 

of  hydra,  123 

sac — megaspore,  293 
Emeu,  650 

Enamel,  of  scales,  348 
Endolymph,  398 
Endoskeleton,   the  internal  skeleton, 

420,  422- 
Endospores,  228 
Endosperm,  53 

gametophyte,  299 
Endothelium — a   thin   layer  of  cells 

lining  a  cavity,  379 
Energy  of  animals,  source  of,  484 

relations  of  the  animal,  446— 
Entamceba,  765 

coli,  510 

histolytica,  510 
Enteropneusta,  618 
Entoderm,  713 
Entomophilous — flowers    which    are 

pollinated  by  insects 
Emydosauria,  644 
Environment,  332 
Ephemeroidea,  584 
Ephyra,  528 

Epidermal  hairs,  of  plants,  21 
Epidermis,  20,  70,  101 

of  leaf,  27,  80 

of  mammal,  343 

of  stem,  73 

of  worm,  340 

origin  of,  715 
Epigynous,  134 

Epiphysis,  the  smaller  bone  formed 
on  the  end  of  a  long  bone, 
with  which  it  ultimately 
unites. 

Epiphytes,  213 
Epithelium,  a  layer  of  cells  covering 

an  organ  or  lining  a  cavity. 
Equilibration,  366 

in  vertebrates,  392-395 
Equisetinae,  287 
Equisetum,  287 
Eustachian  tube,  403 
Euthyneura,  600,  602 
Evolution,  781- 
Excretion,  329,  489- 

by  liver,  493 

in  amoeba,  489 

in  hydra,  489 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


446 


INDEX — GLOSSARY. 


Excretory  organs  of  Nereis,  119,  120 
Exoasci,  26 1 'I 

Exogen,  a  plant  whose  stem  increases 
in  thickness  by  growth  in  the 
region  between  the  bark  and 
the  wood. 
Exoskeleton,  an  external  skeleton. 

of  arthropods,  417 

of  molluscs,  418 

of  vertebrates,  419 
Eye,  control  of  light  intensity  in,  386 

focusing  of,  385 
Eyes,  77,  79,  80 

of  arthropods,  378 

of  cave  fishes,  796 

of  insects,  384 

of  vertebrates,  379 

of  worm,  376,  377 
Eyespot,  in  protozoa,  374 
Eyespots,  526,  528 

F,    FI,    ¥2 — first    filial,    second   filial 

and  third  filial  generations. 
Falcon,  650 

Fascia,  sheets  or  layers  of  connective 
tissue  covering  organs  or 
forming  attachment  for  mus- 
cles, 721 

Fasciola — a  liver  fluke. 
Fats,  473 
Fauna  of  Africa,  826 

of  Australia,  825 

of  caves,  838 

of  fresh  waters,  831-835 

of  oceanic  islands,  827 

of  South  America,  826 
Feathers,  347,  646 
Feigning,  864 
Felidae,  663 
Female,  694 

Ferment,  a  substance  which  causes  a 
chemical  change  in  other  sub- 
stances without  undergoing  a 
permanent  change  itself. 
Fermentation,  230,  453,  670 
Fern,  development  of,  708 

sperms  of,  700 
Ferns,  281 
Ferret,  663 
Fertilization,  135,  187,  694,  698 

in  hydra,  500 

stimulus,  700 
Filament,  123 
Filices,  285 
Filicinae,  281 
Finches,  650 


Fire-fly,  220,  872 

Fire-flies,  rudimentary  wings  of,  797 

Fishes,  glands  of,  350 

respiration  in,  479 

skeleton  of,  424 

sense  of  taste  of,  370 
Fission — reproduction  by  division,  671 
Fissipedia,  663 

Flagellata — Flagellates,  512,  766 
Flagellum — a    whip-like  protoplasmic 

appendage. 
Flame  cell,  534 
Flamingo,  650 
Flat  fishes,  322,  636 

worms,  digestion  in,  454 
Flea,  237,  762 
Flies,  587 

Flight,  adaptation  for,  646 
Floral  structures,  27 
Floridese,  248 
Flounder,  66 

symmetry  of,  322   ' 
Flower,  function  of,  135 

homology  of,  115-120 
Fly  catcher,  650 
Fcetal  membranes,  173,  174,   641 

of  mammals,  651 
Foliate  papillae,  371 
Follicle,  a  minute  cavity,  sac,  or  tube, 

161 

Food  of  animals,  447-451 
Foot,  of  fern,  282,  708 

of  gastropods,  595 

of  molluscs,  592 

of  mosses,  274 

of  rotifers,  539 
Form  of  animals,  308 

of  organisms,  7 
Four-o'clock,  242,  782 
Fowl,  650 

Free  nerve  terminations,  362 
Fresh  water  fauna,  831-835 
Frigate  bird,  650 
Frog,  metamorphosis  of,  211 

respiration  in,  476 
Frogs,  637,  640 
Fruit,  149,  151 
Fruits,  aggregate,  164 

kinds  of,  152-165 

function  of,  168  . 

multiple,  165 

simple,  152-163 
Fucus,  fertilization  of,  693 
Fulgur,  shell  of,  213 
Function  of  the  senses,  405 
Fungi,  177,  179-180 


Numbers  refer  to  paragraphs ;  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


447 


Fungiform  papillae,  371 
Fur  seal,  739 

Gaillardia,  241 
Galeorchis,  849 
Gallinacei,  650 
Galls,  764 
Gall  wasp,  238,  590 
Gametangium  (of  plants) — the  organ  in 
which  gametes  are  produced. 
Gametes,  694 
Gametophyte.  265,  292 
Gamopetalous,  132 
Gamosepalous,  131 
Ganoin,  632 
Garpike,  419 
Garpikes ,  634 
Gastric  fluid,  641 

glands,  720 

mill,  459 
Gastrophilus,  761 
Gastropoda,  595 

structure  of,  595- 
Gastro-vascular  cavity,  454 
Gastrula,  713 

cavity,  713 

mouth,  713 
Gastrulation,  198,  199 
Gavial,  644 
Geese,  650 

Geographical  distribution,  823 
Geotropism,  49,  50 
Germ,  9 

Germinal  tissue,  724 
Germination,  46-50 
Germs,  670 
Gibbons,  668 
Gills,  593,  606 

of  crayfish,  477 

of  fishes,  479 
Gill  slits  of  Dolichoglossus,  618 

of  vertebrates,  813,  814 
Ginkgo,  295 
Ginkgoinae,  295 
Giraffe,  666 
Gizzard,  579 
Glands,  71,  349~3S2 

of  arthropods,  350 

of  birds,  350 

of  fishes  and  amphibia,  350 

of  hydra,  349 

of  mammals,  351 

of  reptiles,  350 
Glandular  activity,  487 
Glossinia,  766 
Glowworm,  739 


Glucose — a    kind   of    sugar   CeHiaOe 

(grape  sugar). 
Glycogen,  472 
Gnats,  587 
Gnetinae,  297 
Goats,  666 

Golden  thread  —  dodder 
Gonad — the  organ  in  which  the  germ 
cells  (gametes)  are  developed. 
Gonads,  of  hydra,  500 

of  insects,  580 

origin  of,  721 
Gordius,  756 
Gorilla,  668 
Grallae,  650 
Grasshopper,  396,  582 
Gray  matter  of  brain  and  spinal  cord, 

444 

Grebes,  650 

Green  gland,  of  crayfish,  491 
Growth,  6,  486 
Grub,  587 
Grubworm,  589 
Guard  cell — one  of  the  epidermal  cells 

which  bound  a  stoma,  86 
Guinea-pig,  660 
Gulls,  650 

Gustatory  sense  —  sense  of  taste. 
Gyncecium,  125,  128 
Gymnophiona,  638 
Gymnospermae — Gymnosperms,  173- 

293 
Gymnotus,  874 

H — symbol  for  hydrogen. 
Haemocyanin,  483 
Haemoglobin,  483 
Hair,  347,  651 

follicle,  70 
Hairs,  of  arthropods,  346 

of  plants,  100 
Halteres,  587 
Hammer,  403 
Harvestmen,  570 

Harvey,  William — English  anatomist 
who  discovered  the  circula- 
tion of  the  blood  (1578-1657), 
670 

Haversian  canals,  426 
Hawks,  650 
Hearing,  366 

and  equilibration,  387- 
Heart,  471,  472 

origin  of,  721- 
Hedgehog,  652 
Heliotropism,  50 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


448 


INDEX — GLOSSARY. 


Heliozoa,  510 
Hellgrammite,  585 
Hemibasidialis,  252 
Hemiptera,  763 

wings  of,  797 
Hepaticae,  267 
Hepatic,  pertaining  to  the  liver. 

caeca,  553 

portal  vein,  the  vein  which  leads 
from  the  intestine  to  the  liver, 
472 
Herbivorous — feeding     on    vegetable 

matter. 
Heredity,  782 

Mendel's  laws  of,  782 

physical  basis  of,  785 
Hermaphrodyte — having  the  organs  of 
both  sexes  in  one  individual, 
502 

Hermit  crab,  562 
Heron,  650 
Herring,  636 
Heterocercal,  633 
Heteronymous,  336 
Heteropoda,  600,  60 1 
Heterotricha,  515 
Hibernia  (moth),  245,  246,  739 

moth,  rudimentary  wings  of,  797 
Hibernation,  842 

"Higher"  and  "lower"  animals,  332 
Hinge  joint,  413 

ligament,  605 
Hippopotamus,  666 
Hirudinea,  546 
H2O  —  symbol  for  water. 
Holocephali,  630 
Holophytic,  744 
Holothuria,  symmetry  of,  320 
Holothuroidea,  556 
Holotricha,  515 
Holozoic,  744 
Hominidae,  669 
Homo,  669 
Homonymous,  336 
Hoofs,  347 
Horns,  347 
Horse,  666 

evolution  of,  248,  807 

-hair  snake,  756 

-shoe  crab,  565 
Host — an   organism   that    harbors   a 

parasite. 
Hosts,  of  bacteria,  773 

alternation  of,  769 
Humming  birds,  650 
Hyaenidae,  663 


Hybrid,  147,  781 
Hydra,  69,  304 

budding  in,  499 

chemical  sense  of,  367 

digestion  in,  454 

excretion  in,  489 

glands  of,  349 

gonads  of,  500 

integument  of,  339 

muscle  fibres  of,  408 

reproduction  in,  499 

response  in,  408 

sense  organs  of,  358 

sensitiveness  to  light,  375 

sexual  reproduction  in,  500 

supporting  lamella,  415 
Hydractinia,  222 
Hydroid,  IM.  222 
Hydrophytes,  103- 192 
Hydropterides,  286 
Hydrotropism,  49 
Hydrozoa,  525 
Hyena,  663 
Hymenoptera,  590 

polymorphism  in,  740 
Hypha — one  of   the  filaments   which 
make  up  the  mycelium  of  a 
fungus. 
Hypocotyl,  52 
Hypoderma,  761 

Hypogynous    (of    stamens,    etc.) — at- 
tached below  the  ovary. 
Hypotricha,  515 
Hyracoidea,  666 
Hyrax,  666 

Ibis,  650 
Ichneumon,  663 

fly,  235, 590, 762 

Imago,  586 
Immunity,  775 

acquired,  777,  780 

natural,  776,  780 

active,  778,  780 

passive,  779,  780 
Impennes,  650 
Incus  —  anvil. 

Indehiscent  (of  fruits) — not  opening. 
Indian  pipe,  751 
Indirect  development,  722 
Individual,  725 
Indusium,  the  membrane  covering  or 

enclosing  a  sorus,  285 
Inferior  (of  ovary) — below  the  calyx, 

134 
Inflorescence,  117-120 


Numbers  refer  to  paragraphs:  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


449 


Ink  gland,  611 
Insecta — insects,  578 
Insects,  development  in,  505 

growth  in,  732 

integument  of,  341 

of  oceanic  islands,  836 

respiration  in,  478 

sense  of  smell  of,  369 
of  taste  of,  369 

voice  of,  396 

wings  of,  412 
Insectivora,  652,  658 
Insertion,  of  muscle,  429 
Inspiration,  481 
Instinct,  875 
Integument,  70,  324,  338 

of  arthropods,  342 

of  insects,  341 

of  mammals,  343 

of  vertebrates,  733 

special  structures  of,  346,  347 
Intelligence,  880 
Internodes — the  portion  of  the  stem 

between  two  nodes. 
Intestinal  glands,  720 
Invertebrates,  eyes  of,  379 
Invertin,  462 
Involucre,  120 
Iris,  379 
Isoetaceae,  291 
Isoetes,  291 
Isopoda,  563 

Jaws,  346 
Jelly  fishes,  528 

nervous  system  of,  437 
Jungermanniaceae,  271 

Kangaroo,  652 
Karyokinesis — mitosis,  686 
Katydid,  396,  582 
Kidneys,  329,  492 
origin  of,  721 
Kidney  tubule,  492 
King  bird,  650 
Kiwi.     See  also  Apteryx,  650 

Labium,  578 

Labyrinth,  392,  393,  398,  399 

Lace  wing  flies,  585 

Lacertilia,  645 

Lacteals,  the  lymphatic  vessels  of  the 

intestine. 

Lacteal  capillaries,  472 
Lactuca,  38,  39 
Lacunae,  426 


Lady  slipper,  850 
Lamellae  of  bone,  426 
Lamellibranchiata,  604 

structure  of,  604 
Lamellirostres,  650 
Lancelet,  625 

egg,  cleavage  of,  712 

notochord  of ,  422 

structure  of,  625 
Lantern  fly,  163 
Lateral  line,  404 
Latitude  and  plants,  204 
Lari,  650 
Larks,  650 
Larvae,  dispersal,  722 
Larva,  the  young  of  an  animal  before 
metamorphosis,  when  devel- 
opment is  indirect. 

of  hydra,  500 

of  the  lancelet,  200,  210 

of  tunicates,  621 

trophic,  723 

Law  of  Biogenesis,  809,  812 
Leaves,  form  of,  23 

storage,  114 

venation  of,  26 

Leaf,  structure  of,  14,  16,  24,  80 
Leech,  locomotion  of,  411 
Legume,  162 
Lemurs,  668 
Lens  of  eye,  381 
Leopards,  663 
Leptocardia,  625 
Lepidoptera,  586 
Leptothrix,  227.  229 
Leptostraca,  561 
Lice,  583 

Lichens,  48,  216,  263,  745 
Life  habits,  744 
Ligaments,  428 

origin  of,  721 

Light  and  plants,  205-208 
Light  sense  in  protozoa,  374 
Lignified  cell- walls,  97 
Lilac  mildew,  748 
Lime,  secretion  of,  349 
Limulus,  151,  565 
Linguatulida,  572 
Linin,  675,  690 
Lions,  663 
Liver,  472,  493,  720 

excretion  by,  493 
Liver  fluke,  231,  232,  754 
Liverworts,  264,  267 
Lizards,  645 

rudimentary  appendages  of,  795 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


45° 


INDEX — GLOSSARY. 


Llama,  666 
Lobster,  561 
Locomotion,  308,  309 

in  annelids,  410,  411 

in  arthropods,  412 

in  vertebrates,  413 
Lophophore,  548 
Love  vine — dodder. 
Luminescence,  production  of  light,  869, 

873 

Luna  moth,  244 
Lungs,  118 

of  crab,  563 

of  dipnoi,  631 

of  snail,  602 

of  vertebrates,  480 
Lycoperdon,  46 
Lycopodiaceae,  289 
Lycopodinae,  288 
Lycopodium,  289 
Lymph,  474 

glands,  origin  of,  721 

origin  of,  721 

spaces,  474 

vessels,  474 
Lysin,  777 

Mackerel,  636 

Macrocystis  pyrifera,  245 

Macrogamete — egg,  694 

Macronucleus — meganucleus,  674,  699 

Macrospore — megaspore. 

Macrura,  561,  562 

Maggot,  587,  670 

Malacostraca,  561,  563 

Malapterurus,  874 

Malarial  fever,  768 

Male,  694 

Males,  dwarf,  739 

Malleus — hammer. 

Malpighian  tubules,  579 

Maltose — a  kind  of  sugar  Ci2H22On. 

Mammalia — mammals,  651 

Mammals  of  Australia,  825 
integument  of,  343,  344 
of  glands  of,  351 

Mammary  glands,  651 

Man,  652 

Manatee,  652,  667 

Mandible,  jaw,  the  lower  jaw  of  ver- 
tebrates or  one  of  the  first 
pair  of  appendages  of  the 
mouth  in  arthropods. 

Mandrels,  668 

Mangrove  seedling,  259 
tree,  854 


Mantle  of  molluscs,  592 

Marchantiaceae,  269 

Marsh  plants,  192 

Marratiacese,  284 

Marsupialia,  652,  654,  655 

Mastigophora,  511 

Maturation,  695 

of  ovum,  184,  185,  785 
of  sperm,  786 

Maxilla,  one  of  the  second  or  third 
pair  of  appendages  of  the 
mouth  of  arthropods,  578, 
586 

Mechanics  of  growth,  212-218,  727 

Mechanism  of  response,  441 

Medullary  groove,  206,  207,  715 
plate,  204,  205,  714,  7i5 
ray,  62 
tube,  209,  715 

Medusa,  525,  528 

Medusae,  light  sense  organs  of,  375 

Meganucleus  —  macronucleus,  the 
larger  of  the  two  nuclei  of  cer- 
tain protozoa. 

Megaspore,  286 

Membrane  bone,  427 

Membrana  tectoria,  398 

Membraneous  labyrinth.  See  laby- 
rinth. 

Mendel's  laws  of  heredity,  782 

Meristematic,  actively  growing. 

Mesenchyma,  mesodermal  tissue  which 
arises  as  separate  cells,  not 
as  a  continuous  layer. 

Mesenteries,  origin  of,  721 

Mesocarpaceae,  237 

Mesoderm,  408,  517,  716,  717,  719 

Mesodermic  somites,  204-210,  717 

Mesoglea,  a  gelatinous  layer  between 
the  ectoderm  and  entoderm 
of  ccelenterates. 

Mesophyll,  27,  8 1 

Mesophytes,  183,  195 

Meso thorax,  578 

Metabolism,  484— 

nuclear  control  of,  682 

Metameres,  334 

Metamorphism,  581 

Metamorphosis — a  marked  change  of 
form  occurring  duringdevelop- 
ment,  320,  506,  637,  723 
in  insects,  505 
in  tunicates,  621 

Metathorax,  578 

Metazoa,  517 

Mice,  652,  660 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


451 


Mice,  inheritance  of  color  in,  783 
Microgamete — sperm,  694 
Micronucleus,  the  smaller  of  the  two 
nuclei    of    certain    protozoa, 
674,  699 

Micropyle,  a  small  hole,  701 
Microsome,  a  small  body,  672 
Microspore — the  smaller  spore,  when 
there  are  two  kinds.     It  gives 
rise  to  a  male  gametophyte, 
286 

Microspora,  growth  of,  728 
Middle  ear,  87,  403 
Migration  of  birds,  843 
Mildew,  257,  748 

Milkweed,  pollination  of,  255,  256,  851 
Mimicry,  865-867 
Mink,  663 

Mirabilis  Jalapa.     See  four  o'clock. 
Mistletoe,  663,  751 
Mitqs,  571 

Mitosis,  180,  181,  479,  686 
Moa,  650 
Mocking  bird,  650 
Modified  roots,  103-106 

stems  and  branches,  107-113 
Moles,  652 
Mollusca,  59_2___ 
Mollusc,  exoskeleton  of,  418 

foot  of,  592 

mantle  of,  592 

segmentation  of,  333 

shell  of,  349,  592 

structure  of,  592 
Molluscoidea,  547 
Monotfemata,  652,  653 
Monkeys,  652,  668 
Monocotyledons,  171,  172,  300 

growth  of,  729 
Monodelphia,  652,  657 
Monodelphous,  133 
Monoecious,  128 
Morchella,  258 
Mosquitoes,  587,  768 
Mosses,  264,  273 

sperms  of,  700 
Mother  of  pearl,  605 
Moths,  586 

Mougeota,  conjugation  in,  693 
Mouth  parts  of  cockroach,  160 

of  insects,  459 
Mucous  epithelium,  720 
Mud  puppies,  639 
Mullet,  636 
Multiple  fruit,  165 
Multicellular  body,  706 


Multipolar  nerve  cells,  435 
Muscle,  325,  429 

fibres,  89-92,  408,  409 
Muscles  of  annelids,  408-411 

of  vertebrates,  414 

origin  of,  721 

stimulus,  431 
Muscular  activity,  488 

contraction,  430 
Mushrooms,  255 

Mycelium — the  mass  of  threads  which 
constitute  the  vegetative  body 
of  a  fungus. 
Mycorhiza,  216 
Mycetozoa,  225 
Mygale,  153 
Myxomycetes,  225 
Myonemes,  88,  407 
Myriapoda,  574 
Myrrh,  261 
Myxamceba,  225 

N — symbol  for  nitrogen. 
Nails,  347 
Narwhal,  665 
Natural  selection,  819 
Nautilus,  1 68,  6n 
Nematoda,  540 
Nemertini,  541 
Nephridia,  329,  593 
Nephridia  of  worms,  490 
Nereis,  67,  305,  543 

locomotion  of,  411 

respiration  in,  476 

sense  organs  of,  360 
Nerve  cells,  97,  100,  101 
of  coelenterates,  435 
types  of,  435  ^ 

-muscle  mechanism,  325 
Nervous  system,  434 

of  annelids,  438 

of  arthropods,  443 

of  dolichoglossus,  618 

of  jelly  fish,  437 

of  nereis,  98 

of  tunicates,  619 

of  vertebrates,  444 
Nettling  cells,  524 
Neuroptera,  585 
Newts,  637,  639 
Nictitating  membrane,  648 
Nitrate  bacteria,  229 
Nitrite  bacteria,  229 
Nitrogen  bacteria,  217 

waste,  488,  490 
Noctiluca,  512 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


452 


INDEX — GLOSSARY. 


Node — the  point  on  a  stem  at  which 

the  leaves  are  borne. 
Non-ruminants,  666 
Notochord,  93,  206^210,422,42^,  716 

of  dolichoglossus,  618 

of  tunicates,  619 
Nuclear  membrane,  674,  685 

sap,  676 

spindle,  683 
Nucleolus,  674 
Nucleoli,  688 
Nucleoplasm,  679 
Nucleus,  1 6,  673 

function  of,  680-682 

resting,  691 

structure  of,  674 
Nut,  158 

Oak  galls,  238 

Oceanic  islands,  fauna  of,  827 
insects  of,  836 

Ocelli,  580 

Octopus,  611,  616.  (See  also  devil- 
fish.) 

Odonata,  584,  585 

(Edogonium,  739 
growth  of,  728 

(Esophagus  of  crayfish,  458 

Oil,  350,  351,  672 

Oils,  vegetable,  94 

Olfactory  sense.     (See  sense  of  smell.) 

Oligochaeta,  545 

Oligotricha,  515 

Ommatidium,  79,  378,  384 

Omnivorous — using  all  kinds  of  foods. 

Ontogeny,  the  history  of  the  develop- 
ment of  the  individual. 

Ontogenetic  series,  809 

Oogonium  (in  the  lower  cryptogams) — 
a  cell  within  which  one  or 
more  ova  are  formed. 

Oomycetes,  250 

Operculum,  279,  632,  633 

Ophidia,  645 

Ophioglossaceae,  283 

Ophiuroidea,  554 

Opilionidea,  570 

Opisthobranchia,  600,  602 

Opossum,  652,  654 

Optic  nerve,  380 

Opuntia,  24 

Orang-utan,  668 

Orchids,  848 

Organ  of  Bojanus,  607 
of  Corti,  85,  86,  398 

Organism,  4 


Organization  of  the  body,  331 
Organs  of  response,  406- 

of  sight,  374- 

of  special  sense,  366- 
Origin  of  species,  789 
Ornithorhynchus.     (See  duck-bill.) 
Orthoptera,  582 
Oscines,  650 
Osmose,  diffusion  of  a  solution  through 

a  membrane,  69,  71 
Osphradium,  an  olfactory  organ  found 

in  many  molluscs,  597 
Ossification  of  vertebra,  217 
Ostium,  an  opening,  mouth,  471,  579 
Ostracoda,  559 
Ostrich,  650 
Otters,  664 
Ovary,  29,  125 

of  hydra,  121,  500 
Ovules,  29,  126 
Ovum  of  hydra,  122,  500 

of  nereis,  125 
Owls,  650 
Oxidation,  475 
Oxygen,  67 

given  off  by  plant,  87 
Oxygenation  of  blood,  482 


Palps,  a  jointed  sensory  organ  attached 

to  the  mouth  appendages  of 

arthropods,  580 
Pain,  364 
Palisade  cells,  81 
Palaeostraca,  564 
Palisade  tissue,  205 
Paludina,  development  of,  805 
Pancreas,  462,  720 
Panther,  663 
Papilio  merope,  267 
Papilionaceous  blossom,  846 
Paramoecium,  TsS-» 

conjugation  in,  186,  699 
Parasites,  215 
Parasitism,  747— 
Parrots,  650 
Parthenogenesis,   development  of   an 

egg  without  fertilization,  581 
Parenchyma,  72 
Passeres,  650 
Pasteur     (Louis),     1822-95,     French 

bacteriologist,   670 
Pearly  nautilus,  611,  616 
Pecora,  666 
Pedicl,  116 
Pedipalpi,  568 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


453 


Peduncle,  116 

Pelican,  650 

Pellicle,  338 

Pelmatozoa,  551 

Penguin,  650 

Pentadactyl  appendages,  637 

Pepsin,  461,  463 

Peptones — soluble  proteid  compounds, 

459 
absorption  of,  472 

Perch,  636 

Perennial,  living  on  from  year  to  year. 

Perianth,  122 

Pericardial  cavity,  471 

Pericardium,  origin  of,  721 

Pericarp — the  wall  of  the  ovary  when 
the  fruit  is  mature. 

Peridermium,  227,  228 

Perigynous,  134 

Perilymph,  399 

Peripatus,  157 

Perisarc,  339 

Perisperm,  53 

Perisporeaceae,  257 

Perissodactyla,  666 

Peris  tome,  247 

Periostracum,  605 

Perithecium — (of  fungi)  an  urn-shaped 
fruiting  receptacle,  257 

Peritoneum,  origin  of,  721 

Peritricha,  515 

Petiole,  24,  25 

Petals — the  parts  of  the  corolla,  132 

Phaeophyceae,  245 

Phaeosporeae,  246 

Phagocytosis,  the  destruction  of  bac- 
teria by  the  white  blood  cor- 
puscles— phagocytes,  776 

Phanerogams,  flowering  plants,  292 

Pharynx,  of  tunicates,  619 

Phascaceae,  278 

Pheasant,  650 

Phenacodus,  666,  807 

Phloem,  the  part  of  the  vascular 
bundle  containing  the  sieve 
tubes. 

Phosphorescence — luminescence. 

Phosphorus,  679 

Photogenic — producing  light. 

Photosynthesis — the  process  by  which 
organic  substances,  such  as 
starch,  are  formed  by  the 
agency  of  chlorophyll  in  sun- 
light, 87 

Phycocyanin,  232 

Phycoerythrin,  248 


Phycophaein — a  brown    pigment  con- 
tained   in    the    brown     sea- 
weeds, 245 
Phycomycetes,  249 

Phyllotaxy — leaf  arrangement,  -30-33 
Phyllopoda,  559 
Phylloxera,  763 

Phylogey,  the  history  of  the  develop- 
ment of  the  race. 
Phylogenetic  series,  802 
Physalia,  134 
Physiographic  relations  of  plants,  219- 

223 

Pici,  650 
Pigeon,  650 

Pigment  layer  of  eye,  379 
Pike,  636 
Pill  bug,  561 
Pine,  35 
Pineal  eye,  796 
Pinnipedia,  664 
Pisces,  628 

structure  of,  628 
Pistil,  29,  125 

Pistil — megasporophyll,  298 
Pith,  6 1 
Placenta,  126 
Plant  bugs,  591 
Plants,  color  of,  18-22 
Plant  hairs,  100 
Plantigrade — standing  on   the   whole 

sole  of  the  foot,  663 
Plant  lice,  591,  763 
Plant  series,  793 
Planula,  528 

Plasmodium — the  amceboid   stage   of 
the  body  of  a  slime  mold, 
252,  240,  768 
Plastron,  643 
Platyhelminthes,  534 
Platyrhina,  668 

Pleura,  the  membrane  covering  the 
lungs  and  lining  the  thoracic 
cavity. 

origin  of,  721 
Pleurobrachia,  531 
Plumule,  55 
Pocket-gopher,  660 
Poison  gland,  567,  569 
Polar  body,  696 
Pole  cat,  663 
Pollen,  123 

grain — microspore,  293 

protection  of,  137 

selection  of,  147 

tube  nucleus,  294 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer-to  figures. 


454 


INDEX — GLOSSARY. 


Pollination,  135-147,  845 

by  insects,  140,  143 

by  wind,  139 
Polychaeta,  544 
Polygala,  30,  31 
Polymorphism,  221-223,  527,  458,  581, 

583 

in  bryozoa,  740 
in  hydrozoa,  740 
in  plants,  743 
Polyps,  525 
Polypodium,  40,  41 
Polyprotodontia,  652,  655 
Polyzoa,  548 
Porifera,  519- 
Porpoises,  652 
Posterior,  311 
Prawns,  561 
Praying  mantis,  582 
Primates,  652,  668 
Proboscidia,  666 
Proboscis,  586 
Procyonidae,  663 

Proglottis,  segment  of  a  tapeworm,  537 
Pronuba,  852,  853,  875 
Prosimiae,  668 
Protective  coloration,  861,  862 

in  grasshoppers,  262,  263 
resemblance,  264,  863 
Proteolytic — having  the  property  by 

which  proteids  are  changed 

into  peptones,  461 
ferments,  463 
Prothallium,  the  thalloid  gametophyte 

of  certain  pteridophytes,  280 
Pro  thorax,  578 
Protobasidiomycetes,  254 
Protoascales,  240 
Protonephridia,  534,  541 
Protonema,  the  green  thread-like  part 

of  the  gametophyte  of  mosses, 

275 
Protoplasm,  13 

chemical  properties  of,  15,  68,  672, 
680 

contractility  of,  680,  68 1 

irritability  of,  680 

structure  of,  178,  672 
Protozoa,  507- 

as  parasites,  765 

conjugation  in,  498 

sensitiveness  to  light,  355 
Protracheata,  573 
Pseudopodium  276 
Psittaci,  650 
Pteridophyta-Pteriodophytes,  175, 280 


Ptyalin,  461 

Puff  balls,  255 

Pulmonata,  600,  602 

Pupa,  586,  723 

Pupil,  379 

Pygopoda,  650 

Pyrenomycetes,  259 

Python,  rudimentary  appendages,  795 

Quail,  650 

Queen  conch,  shell  of,  218 

Rabbits,  652,  660 

Raceme,  119 

Racemose  inflorescence,  26 

Raccoon,  663 

Radial  symmetry,  316 

Rachis,  29 

Radiolaria,  510 

Radula,  593,  596 

Rails,  650 

Ratitae,  650 

Rats,  652,  660 

Ravens,  650 

Receptacle,  122,  130 

Receptors,  439,  441 

Recessive  characters,  in  heredity,  783 

Red  blood  corpuscles,  483 

Redi,     Francesco,     1626-98,     Italian 

naturalist,  670 
Redii,  754 

Reduction  division,  695,  696,  698 
Regeneration,  726 
Regions  of  growth,  102 
Reproduction,  330,  495- 

in  amoeba,  495,  496 

in  annelids,  501,  502 

in  crayfish,  503,  504 

in  hydra,  499 

in  sponge,  523 

in  vertebrates,  506 
Reptilia — reptiles,  691 
Reptiles,  glands  of,  350 

structure  of,  641 
Reserve  food,  672 
Resemblance,  protective,  863 
Respiration,  308,  327,  475- 

in  amoeba,  476 

in  crayfish,  477 

in  earthworm,  476 

in  fishes,  479 

in  frog,  476 

in  insects,  478 

in  nereis,  476 

in  plants,  91 

in  sea  anemone,  476 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


455 


Respiratory  organs  of  crayfish,  116, 117 
of  nereis,  115 

tree,  556 
Response,  in  amoeba,  407 

in  annelids,  409-411 

in  hydra,  408 
Retina,  379 

Retinal  elements,  81,  380 
Rhabdom,  384 
Rhea,  650 
Rhinoceros,  666 
Rhizoids.  267 
Rhizopoda,  509 
Rhodophyceae,  248 
Rhomboganoidea,  634 
Rhythmical  changes  in  plants    197, 

199  200 

Rhynchocephalia,  642 
Rhynchota,  585,  591 
Ricciaceae,  268 
Rice  birds,  650 
Rocks,  decay  of,  222 
Rodentia,  652,  660 
Rods,  of  retina,  380 
Root,  7 

cap,  102 

hairs,  8,  68-69 

tip,  23 
Roots,  37,  39 

aerial,  105 

of  epiphytes,  106 

prop,  104 

storage,  103 

structure  and  function  of,  68-  71 
Rootstock,  in 
Rosette  habit,  201 
Rotatoria,  539 
Rotifers,  foot  of,  539 
Round-mouthed  eel,  423,  627,  752 
Round  worms — thread  worms,  540 
Rudimentary  organs,  794— 
Ruminants,  666 
Runner,  109 
Rusts,  254,  749 

S — symbol  for  sulfur. 
Saccharomycetes,  262 
Sacculina,  560,  739,  760 
Sacculus,  393,  394 
Salamanders,  637,  639 
Salamandra,  171 
Salicornia,  37 
Salivary  glands,  459,  720 

of  insects,  579 

of  man,  461 

of  nereis,  455 


Salmon,  636 

Salvia,  461,  847 

Samara,  159 

Sand  flea,  561 

Saprophytes,  214 

Saprozoic — nourished  like  fungi,  i.e.,  on 

soluble  organic  matter. 
Sarracenia,  53-55 
Scale  insects,  591,  763 
Scale  leaves,  114 
Scales,  70 

bony,  348 

of  vertebrates,  347 

of  worms,  346 

Scaly  ant  eater,  421,  652,  66 1 
Schizocarp,  160 
Schizophyceae,  232 
Schizophyta,  226 
Schizopoda,  561 
Sclera,  379 
Sclerenchyma — hard  tissue,  cells  with- 

thick  walls. 
Scolecida,  533 
Scolex,  537,  755 
Scorpion,  152 
Scorpionidea,  567 
Scyphozoa,  528 
Sea  anemone,  529 

chemicatfsense  organs  of,  367 
respiration  in,  476 

cow,  667 

cucumber,  556 

horse,  636 

lilies,  551 

squirt,  621 

turtles,  643 

urchins,  555 

weeds,  208 
Seal,  652,  661 
Seed,  the,  148,  151,  292 

distribution,  166-168 
Seedling,  51 
Seeds,  42-45 
Segmentation,  of  body,  333,  337 

of  egg — cleavage,  190-195,  702 
Selachii,  629 
Selaginella,  290 
Selaginellaceae,  290 
Self  fertilization,  144 
Semicircular  canals,  393,  394 
Sense  of  position,  365 

of  smell,  366 
of  insects,  369 
of  man,  373 
of  vertebrates,  370 

of  taste,  366 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


456 


INDEX — GLOSSARY. 


Sense  of  taste  of  fishes,  370 
of  insects,  369 
of  man,  372 
of  vertebrates,  370 
of  touch,  364 

of  amoeba,  354 
of  weight,  365 
organs,  77,  325,  353- 
of  amoeba,  353~357 
of  arthropods,  361 
of  ccelenterates,  359 
of  hydra,  358 
of  Nereis,  360 
of  vertebrates,  362 
special,  311 
Senses  of  animals,  404 
Sensibility,  309 
Sensory  corpuscles,  362 
Sepal — one  of  the  parts  of  the  calyx,  131 
Serpent  star,  554 
Sessile — not  raised  on  a  stalk. 
Seta,  279,  346 
Sex,  development  of,  182 
Sexual  dimorphism,  219,  220 
reproduction  in  hydra,  500 
selection,  857 
Sharks,  skeleton  of,  423 
Sheep,  666 

Shell  of  animals,  growth  of,  730 
of  gastropods,  595 
of  molluscs,  418,  592 
of  turtles,  420 
Shrew,  652 
Shrimp,  149,  561 
Sieve  tubes,  n,  74 
Sight,  366 
Simise,  668 
Siphon,  605 
Siphonales,  243 
Siphonaptera,  588 
Siphonocladiales,  242 
Siphonophore,  221 
Sirenia,  652,  667 
Sirenidae,  639 
Sitaris,  876 
Size  of  organisms,  limitation  of,  8 

and  differentiation,  323 
Skeletal  muscles,  414 
Skeleton,  325 

and  connective  tissue,  415 
cartilaginous,  423,  424 
of  sponge,  520 

of  vertebrates,  growth  of,  734,  735 
Skull  of  human  embryo,  216 

of  vertebrates,  792 
Slaves  of  ants,  590 


Sleeping  sickness,  766 
Slime  mold,  57 

molds — myxomycetes. 
Sloth,  652 
Slug,  165 
Slugs,  602 
Smell,  366 

Smooth  muscle  fibre,  414 
Smut,  253,  259 
Snail,  164,  165 
Snails,  595 

fresh  water,  602 
Snakes,  645 
Snapping  turtle,  643 
Soil  and  plants,  209-210 

bacteria,  217 
Soldier  (of  Aphids),  583 
Solenoconchae,  603 
Sole,  symmetry  of,  322 
Somatic  tissue,  724 
Sorus,  a  cluster  of  sporangia,  284,  285 
South  American  fauna,  826 
Sparrows,  650 
Species,  781 

number  of,  788 

origin  of,  789- 
Spermatia,  248 
Spermatogenesis,  183,  185 
Spermatophyta — spermatophytes, 

170,  292 
Sperm  cells,  formation  of,  695 

motility  of,  697 

nucleus,  698,  699 
of  hydra,  500 

nuclei  of  pollen,  299 
Sphagnacese,  276 
Sphex,  876 
Spicules,  522 
Spider,  569 

web,  844 

Spiders,  polymorphism  in,  743 
Spike,  119 
Spinal  cord,  103,  444 

ganglia,  363 

nerves,  444,  445 
Spindle  fibres,  690 
Spiny  ant-eater,  652 
Spiral  valve,  627,  629,  631,  632 
Spirillum,  227 
Spirobolus,  158 
Spirochaete,  227 
Spirogyra,  conjugation  of,  693 
Spirostomum,  myonemes  of,  88 
Spleen,  origin  of,  721 
Sponge,  129-132 

structure  of,  520 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


457 


Sponges,  519- 

canal  system  of,  521 

digestion  in,  454 

reproduction  in,  523 
Spongilla,  519 

Spontaneous  generation,  670 
Spore,  181,  671 
Sporidia,  749 

Sporophyll,  a  leaf  bearing  spores,  282 
Sporophyte,  the  generation  which  pro- 
duces spores  asexually,  but  is 
produced  sexually,  265 
Sporozoa,  513,  768,  774 
Spring  lizards,  639 

tails,  577 
Spurs,  347 
Squamata,  645 
Squash  bug,  591 
Squid,  611,  616 
Squirrels,  652,  660 
Stapes  —  stirrup,  399 
Staphylococcus,  231 
Starch,  18,  87,  672 

changed  to  sugar,  90 
Starfish,  141-143,  MS,  55^553 
Statocysts,  83,  387-528,  531 
Statolith,  388,  390 
Stamen,  123,  124 
Steapsin,  462 
Steganopodes,  650 
Stem,  34,  36 

section  of,  10,  12,  13 
"Stemless"  plants,  107 
Stems  of  biennials,  107 

of  climbers  and  trailers,  108 

storage,  112 

structure  and  function  of,  72-78 
Sterculia,  28 

Sterigma — a  stalk  (of  a  spore). 
Stigma  (of  plants),  125,  127 
Stigmata  (of  insects),  478,  574 
Sting,  590 
Stirrup,  403 
Stolon,  109 

Stoma,  pi.  stomata,  15,  17,  80 
Stomach  of  crayfish,  458 

of  insect,  579 
Stomata  of  aquatics,  188,  189 

function  of,  84 
Stomatopoda,  561 
Storage  stems,  no 
Stork,  650 
Stormy  petrel,  650 
Streptococcus,  231 
Strep toneura,  600,  60 1 
Striate  muscle  fibres,  414 


Striges,  650 

Strobila — a  chain  of  segments  or  indi- 
viduals formed  by  transverse 
division  of  the  parent  organ- 
ism, 124,  755 

Strobila  of  microstomum,  124 
Struggle  for  existence,  816 
Struthiomorphae,  650 
Sturgeons,  633 
Style,  125 

Suberized  cell  walls,  97 
Subterranean  stems,  202 
Suctoria,  516 

Sugar,  C6H1206,  Ci2H22On,  90 
Superior     (of    the    ovary) — attached 

above  the  calyx. 
Susceptibility,  774 
Suspensor,  289,  290,  294,  709 
Swallows,  650 
Swamp  plants,  192 
Swarm  spore — a  ciliated,  motile  spore. 

conjugation  of,  693 
Sweat  glands,  352 
Swine,  666 
Symbionts,  216 
Symbiosis,  745,  746 
Symmetry,  314 

of  echinoderm  larvae,  320 

of  gastropods,  319 

universal,  317 
Synergids,  299 
Syphostoma,  528 
Syrinx,  647 

Tachyglossus.     See  anteater. 
Tadpole  of  tunicates,  621 
Taenia — a  tapeworm. 
Tapeworm,  233,  234  755 
Tapir,  666 
Taste,  366 

buds,  77,  371 
Taxonomy,  systematic  classification  of 

organisms. 

Taxonomic  series,  790,  791 
Teeth,  348 

of  mammals,  651 

origin  of,  721 
Tegmen,  42 
Teleostei,  636 
Teleutospores,  749 
Temperature  and  vegetation,  196-203 

control,  840,  841 

sense,  364 

of  amoeba,  356 
Tendons,  429 

origin  of,  721 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


458 


INDEX — GLOSSARY. 


Tendrils,  114 
Tentacles,  528 

Termes — a  genus  of  termites. 
Terminal  bud,  55 
Termite  queen,  260 
Termites,  223,  224,  583 
polymorphism  in,  740 
Terrapin,  643 

diamond  back,  215 
Testa,  42 

Testes  of  hydra,  121,  500 
Testudinata,  643 
Tethyodea,  621 
Tetrabranchiata,  616 
Tetrads  (in  maturation),  695 
Tetraspores — four     spores     produced 

asexually  in  one  mother  cell, 

248 

Texas  cattle  fever,  771 
Thaliaceae,  623 
Thallophytes,  177,  224-263 
Thoracic  duct,  472,  473 
Thoracostraca,  561 
Thorns,  113,  114 
Thousand  legs,  574,  575 
Thread  worms,  540,  756-759 
Thrushes,  650 
Thymus  gland,  720 
Thyroid  gland,  720 
Ticks,  571 
Tigers,  663 
Tillandsia,  45 
Tinamiformes,  650 
Tissues,  origin  of,  719 
Titmouse,  650 
Toad  fish,  636 
'Toads,  637,  640 
Torpedo,  874 
Tortoise,  643 

^shell,  347,  643 
Toxin,  776 
Tracheae,  n,  569,  574,  579 

of  insects,  162,  478 
Tracheids,  n,  74,  296 
Traguloidea,  666 
Translocation  of  food  substances,  92, 

93 

Transpiration,  86 
Trap-door  spider,  154-156 
Tree  toads,  640 

Trematoda — Trematodes,  536,  753 
Trichinella,  757 
Trichogyne,  248 
Trochophore,  609 

larva  of  annelids,  501 
Tropaea.     See  luna  moth. 


Trunk  fish,  419,  636 
Trypanosome,  239,  766,  767 
Trypsin,  462,  463 
Tryptic — having  the  proteolytic  action 

of  trypsin. 
Tsetse  fever,  766 

fly,  766 
Tube  feet,  550 
Tuber,  no 
Tuber,  47 
Tuberaceae,  260 
Tubinares,  650 
Tunic,  619 
Tunicata,  619 
Turbellaria,  535 
Turgid,  distended,  swollen. 
Turkey,  650 
Turtles,  643 

shell  of,  420 
Tylopoda,  666 
Tympanum — eardrum,  397,  401 

Ulothricales,  241 
Umbel,  119 
Ungulata,  652,  666 
Urea,  COH4N2,  492 
Uredospores,  749 
Ureter,  492 
Urinary  bladder,  492 
Ursidae,  663 
Urochorda,  619 
Urodela,  172,  639 
Uropygal  gland,  350 
Utriculus,  393,  394 

Vacuole,  672 
Vascular  bundle,  74 
Variation,  781 

Variety — subdivision  of  a  species,  781 
Vegetation  and  climate,  223 
Veliger,  a  mollusc  larva  of  peculiar 
form  with   a   ciliated   collar, 

598 
Ventral,  312 

ganglionic  chain — ventral  nerve 
cord. 

nerve  cord,  438,  580 
Vertebral  column  of  vertebrates,  792 
Vertebrata — vertebrates,  480,  626 
Vertebrate  appendage,  413,  799 

eye,  379 
Vertebrates,  circulation  in,  472 

development  in,  506 

digestive  tract  of,  460 

exoskeleton  of,  419 

kidneys  of,  492 


Numbers  refer  to  paragraphs;  Black  Face  numbers  refer  to  figures. 


INDEX — GLOSSARY. 


459 


Vertebrates,  locomotion  in,  413 

nervous  system  of,  444 

reproduction  in,  506 

segmentation  of,  336 

sense  of  smell  of,  370 
of  taste  of,  370 
organs  of,  362 
Vessels  (of  plants),  74 
Vestigeal  organs,  794- 
Vibrio,  227 
Viffi,  472 
Visceral  sac,  595 
Vision,  383,  384 
Vitreous  humor,  382 
Viverridae,  663 
Voice,  396 
Volvocales,  239 
Vultures,  650 

Walking  stick  insect,  265 

Walrus,  664 

Warblers,  650 

Warm  blooded  animals,  840 

Wasps,  590 

care  of  young,  855 
Water  and  vegetation,  183-195 

boatmen,  591 

bugs,  591 

striders,  591,  763,  766 

vapor,  transpiration  of,  84 
Weasel,  663 
Web  of  spider,  569 
Whales,  652,  665 
Whale  bone,  250,  347 

rudimentary  limbs  of,  795 
Wheat  rust,  749 
"White  ants"  —  termites,  583 
White  blood-corpuscles  —  leucocytes, 

494,  7?6 
Numbers  refer  to  paragraphs;  Black 


Whip-poor-will,  650 
Whorl — three  or  more  leaves,  or  other 
parts,   set  around  the  same 
node. 

"Witches  broom,"  261 
Wolves,  663 
Wood  lice,  561,  563 

pecker,  650 

Worker  (ants  or  termites),  583 
Worms,  349,  542 

connective-tissue  of,  416 

digestion  in,  455~4S7,  466 

eyes  of,  376 

glands  of,  349 

integument  of,  340 

nephridia  of,  490 

sense  organs  of.     See  nereis. 

sensitiveness  to  light,  376 

Xerophytes,  183,  193-195 
Xiphosura,  565 

Xylem,  the  wood  portion  of  a  vascular 
bundle. 

Yeast,  262 

Yolk,  effect  on  cleavage,  711 
Young,  care  of,  854 
Yucca,  852 

pollination  of,  257,  258 

Zooglea,  228 
Zygnemaceae,  236 
Zygomycetes,  250 

Zygospore,    a    spore    formed   by    the 
fusion  of  two  similar  gametes, 

234 

Zygote,  a  body  formed  by  the  fusion  of 
two  gametes. 

Face  numbers  refer  to  figures. 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

C«P«L 

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Renewed  books  are  subject  to  immediate  recall. 


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